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### Chap07 e

1. 1. SimilarityIt Really Is a Small World In California, there is a theme park basedon a popular brand of plastic building blocks. One of the greatattractions at the park is a detailed scale model of five areas ofthe United States—Washington, D.C., New Orleans, New York,New England, and the California coastline—built from20 million plastic blocks. This attraction was created tocelebrate the diversity of the United States and thepeople who live there.Think About It The model of the U.S. Capitolbuilding has a height of 7.2 feet—measured fromits base to the top of the statue of freedom. Theactual Capitol building is 40 times as tall. Howtall is the actual Capitol building?448-449_07elMSMgr7 1/6/04 9:26 AM Page 448
2. 2. Family LetterDear Student and Family Members,Our next chapter is about similarity between figures or shapes. Similarfigures have the same shape, but not necessarily the same size. Congruentfigures are figures that have the same shape and the same size. We willexplore similar geometric figures in two-dimensional and three-dimensionalmodels, and learn about scale and scale factors between similar figures.We will also investigate the relationships between the scale factor, area, andperimeter of similar figures. We will learn how to dilate two-dimensionaldrawings as well as three-dimensional objects. For instance, model trains arescale model of real trains and a globe is a scale model of Earth.At the end of the chapter, we will apply what we’ve learned to solve aninteresting problem: Could the bones of giants 12 times as large as we arereally support their weight? Make a prediction and compare it to the answerat the end of the chapter.Vocabulary Along the way, we’ll be learning about these new terms:congruent counterexample ratiocorresponding angles dilation scale factorcorresponding sides equivalent ratios similarWhat can you do at home?As you look around, you are likely to see many examples of similar shapes:scale drawings, reduced or enlarged photocopies, maps and the actual areasthey show, and different-sized boxes of the same kind of cereal or other items.It might be fun for you and your student to point out what you think are simi-lar shapes, and then measure each to verify that they actually are similar.impactmath.com/family_letter 449IMNOPJKLCongruent Figures Similar Figures448-449_07elMSMgr7 6/7/04 4:25 PM Page 449
3. 3. 450 C H A P T E R 7 SimilarityWhat does it mean to say that two figures are the same?These figures are “the same” because they are members of the same classof objects. They are both rectangles.Some figures have more in common than just being the same type of fig-ure. One of the figures below, for example, is an enlargement of the other.They have the same shape but are different sizes. Two figures that havethe same shape are similar.Of course, the most obvious way in which two figures can be “the same”is for them to be identical. Figures that are the same size and the sameshape are congruent. The figures below are congruent.These rectangles are also congruent.Notice that similarity and congruence don’t depend on how the objectsare positioned. They can be flipped and rotated from each other.Are Theythe Same?V O C A B U L A R YsimilarV O C A B U L A R Ycongruent450-517_07elMSMgr7 1/4/04 9:12 AM Page 450
4. 4. InvestigationL E S S O N 7 . 1 Are They the Same? 451ExploreYour teacher will give you a sheet of paper with drawings of threefigures. One or two other students in your class have figures that arecongruent to yours. Find these students.How did you determine which of the other students’ figures werecongruent to yours?11 Identifying Congruent Figuresand AnglesTo find who had figures congruent to yours, you needed to invent a wayto tell whether two figures are congruent. Now you will use your test forcongruence on more figures and on angles.Problem Set AIn each problem, decide whether Figures A and B are congruent. If theyare not congruent, explain why not.1. 2.3.4.ABABABA BM A T E R I A L S• ruler• protractor450-517_07elMSMgr7 1/4/04 9:12 AM Page 451
5. 5. &452 C H A P T E R 7 Similarity5. 6.In Problem Set A, you compared several pairs of figures. One importantgeometrical object is a part of many figures: the angle. What do you thinkcongruent angles look like?Think DiscussThe angles in each pair below are congruent.What do you think it means for angles to be congruent?Problem Set BDecide whether the angles in each pair are congruent. If they are notcongruent, explain why not.1. 2.cdabABABM A T E R I A L S• ruler• protractor450-517_07elMSMgr7 1/4/04 9:13 AM Page 452
6. 6. &L E S S O N 7 . 1 Are They the Same? 453One way to test whether two figures are congruent is to try fitting oneexactly on top of the other. Sometimes, though, it’s not easy to cut outor trace figures, so it’s helpful to have other tests for congruency.Problem Set CEach problem below suggests a way to test for the congruence of twofigures. Decide whether each test is good enough to be sure the figuresare congruent. Assume you can make exact measurements.If a test isn’t good enough, give a counterexample—that is, an examplefor which the test wouldn’t work.1. For two line segments, measure their lengths. If the lengths areequal, the line segments are congruent.2. For two squares, measure the length of one side of each square. Ifthe side lengths are equal, the squares are congruent.3. For two angles, measure each angle with a protractor. If the angleshave equal measures, they are congruent.4. For two rectangles, find their areas. If the areas are equal, the rectan-gles are congruent.ShareSummarizeDecide which figures in each set are congruent. Explain how you know.1.2.ijklM NPRQM A T E R I A L S• ruler• protractorV O C A B U L A R YcounterexampleM A T E R I A L S• ruler• protractor450-517_07elMSMgr7 1/4/04 9:13 AM Page 453
7. 7. Investigation454 C H A P T E R 7 Similarity22 Are They Similar?You now have several techniques for identifying congruent figures. Howcan you tell whether two figures are similar?Problem Set DWork with a partner. To begin, draw arectangle with sides 1 cm and 3 cm long. Youneed only one rectangle for the two of you.1. Now, one partner should draw a new rectangle whose sides are7 times the length of the original rectangle’s sides. The other partnershould draw a new rectangle in which each side is 7 cm longer thanthose of the original rectangle. Label the side lengths of both newrectangles.2. With your partner, decide which of the new rectangles looks similarto the original rectangle.In Problem Set D, you modified a figure in two ways to create largerfigures. Now you will compare two ways for modifying a figure to createsmaller figures.Problem Set EWork with a partner. Begin by drawing a rectangle with sides 11 cm and12 cm long.1. Now, one partner should draw a new rectangle whose sides are one-tenth the length of the original rectangle’s sides. The other partnershould draw a new rectangle in which each side is 10 cm shorter thanthose of the original rectangle. Label the side lengths of both newrectangles.2. With your partner, decide which of the new rectangles looks similarto the original rectangle.1 cm3 cmM A T E R I A L Smetric rulerRememberFigures are similar ifthey have the exactsame shape. They maybe different sizes.M A T E R I A L Smetric ruler450-517_07elMSMgr7 1/4/04 9:13 AM Page 454
9. 9. Investigation456 C H A P T E R 7 Similarity33 Ratios of Corresponding SidesA ratio is a way to compare two numbers. When one segment is twice aslong as another, the ratio of the length of the longer segment to the lengthof the shorter segment is “two to one.”One way to write “two to one” is 2:1. That means that for every 2 units oflength on the longer segment, there is 1 unit of length on the shorter seg-ment. For example, the ratio of the length of Segment m to the length ofSegment k is 2:1.Two other ways to write “two to one” are “2 to 1” and ᎏ21ᎏ.It’s possible to use different ratios to describe the same relationship.mkV O C A B U L A R YratioMaya and Simon think about the ratios of the side lengths in triangles differently.I can comparethe sides oftwo trianglesby using ratios.The ratio ofthe length ofSide AB tothe length ofSide WX is 1:3 because Imultiply thelength ofSide AB by 3 toget the length ofSide WX. In otherwords, for every1 cm of Side ABthere are 3 cmof Side WX.I think about it differently.Since Side AB is 4 cm longand Side WX is 12 cm long,the ratio of the length ofSide AB to the length ofSide WX is 4:12.For every 4 cmon Side AB,there are 12 cmon Side WX.AB C5 cm3 cm4 cmWYX15 cm9 cm12 cmE X A M P L EE X A M P L E450-517_07elMSMgr7 1/4/04 9:14 AM Page 456
10. 10. L E S S O N 7 . 1 Are They the Same? 457Two ratios are equivalent ratios if they represent the same relationship.Maya pointed out that the ratio 1:3 means that for every 1 cm of length onone segment, there are 3 cm of length on the other. Simon said the ratio4:12 means that for every 4 cm of length on one segment, there are 12 cmof length on the other. These two ratios represent the same relationship:the length of the first segment is multiplied by 3 to get the length of thesecond segment. Therefore, 1:3 and 4:12 are equivalent ratios.Problem Set G1. Name at least two ratios equivalent to the ratio of the length ofSegment MN to the length of Segment OP.Decide whether the ratios in each pair are equivalent. Explain howyou know.2. 1:4 and 2:8 3. ᎏ25ᎏ and ᎏ39ᎏ4. 3:5 and 5:3 5. ᎏ13ᎏ:1 and 1:36. Darnell and Zoe were analyzing a pair of line segments. “Thelengths are in the ratio 2:3,” Darnell said. “No,” Zoe replied, “theratio is 3:2.” Their teacher smiled. “You’re both right—but to beclear, you need to give more information about your ratios.”What did their teacher mean? Are 2:3 and 3:2 equivalent? Howcould Darnell and Zoe both be correct?In Investigation 2, you created rectangles and triangles that were similarto other rectangles and triangles. For each shape, you used a part of theoriginal figure to create the corresponding part of the new figure.Corresponding parts of two similar figures are located in the same place ineach figure. For example, Triangles ABC and DEF are similar. Sides AB andDE are corresponding sides, and ЄB and ЄE are corresponding angles.EFDBCAMNOPV O C A B U L A R Yequivalent ratiosV O C A B U L A R Ycorrespondingsidescorrespondingangles450-517_07elMSMgr7 1/4/04 9:15 AM Page 457
11. 11. 458 C H A P T E R 7 SimilarityWhen you created similar rectangles and triangles, the ratios of thelengths of each pair of corresponding sides were equivalent. In fact,this is true for all similar figures: the ratios of the lengths of each pairof corresponding sides must be equivalent.Problem Set HThe figures in each pair are similar. In each problem, identify all pairs ofcorresponding sides and all pairs of corresponding angles.1. 2.3. OTGADCECFDA BETCNOULMM A T E R I A L S• ruler• protractorThe concept of similartriangles can be usedto estimate dimen-sions of lakes, heightsof pyramids, anddistances betweenplanets.factsJustt h e450-517_07elMSMgr7 1/4/04 9:15 AM Page 458
12. 12. L E S S O N 7 . 1 Are They the Same? 459Problem Set IIf figures are similar, each pair of corresponding sides must have the sameratio. But if ratios of corresponding sides are the same, does that mean thefigures must be similar? You will explore this question now.1. Here are two quadrilaterals.a. Copy and complete the table for Quadrilateral DEFG.Description Side Length (cm)longest side DGsecond-longest side FGthird-longest side DEshortest side EFb. Now complete the table for Quadrilateral WXYZ.Description Side Length (cm)longest side XYsecond-longest side WXthird-longest side YZshortest side ZWc. Find the ratio of the longest side in Quadrilateral WXYZ to thelongest side in Quadrilateral DEFG. Find the ratios of the remain-ing three pairs of sides in the same way:• second longest to second longest• third longest to third longest• shortest to shortestd. What do you notice about the ratios in Part c? Are QuadrilateralsWXYZ and DEFG similar? Explain your answer.GDEFZWYXM A T E R I A L Smetric ruler450-517_07elMSMgr7 1/6/04 7:32 AM Page 459
14. 14. InvestigationL E S S O N 7 . 1 Are They the Same? 46144 Identifying Similar PolygonsIn Investigation 3, you found that when two figures are similar, the ratio oftheir corresponding side lengths is always the same. Another way of sayingthis is that the lengths of corresponding sides share a common ratio.You also discovered that angles are important in deciding whether twofigures are similar. However, you might not have found the relationshipbetween corresponding angles. In fact, for two polygons to be similar,corresponding angles must be congruent. You won’t prove this fact here,but you will use it throughout the rest of this chapter.To test whether two polygons are similar, you need to check only thatcorresponding side lengths share a common ratio and that correspondingangles are congruent.Problem Set JDetermine whether the figures in each pair are similar. If they are not sim-ilar, explain how you know.1.2.3.F LE DK JC IBAHGIMNOPJKLA BD CE FH GRememberTwo angles are congru-ent if they have thesame measure.M A T E R I A L S• ruler• protractor450-517_07elMSMgr7 1/4/04 9:16 AM Page 461
15. 15. 462 C H A P T E R 7 Similarity4.5.If two figures do not have line segments and angles to measure, how canyou decide whether they are similar? One method is to check importantcorresponding segments and angles, even if they are not drawn in. Forexample, on these two spirals, you might measure the widest and tallestspans of the figures (shown by the dashed segments) and check whetherthey share a common ratio.Just checking these two segments won’t tell you for sure that the figuresare similar, but it will give you an idea whether they could be. If the ratiosaren’t equivalent, you will know for certain the figures are not similar.EGHFDKLMNJST YZXQWR450-517_07elMSMgr7 1/4/04 9:16 AM Page 462
16. 16. &L E S S O N 7 . 1 Are They the Same? 463Problem Set KWork with a partner. Try to figure out whether each pair of figures is, orcould be, similar. Explain your decisions.1. 2.3.ShareSummarizeTry to stump your partner by drawing two pentagons, one that’s similarto this pentagon and another that isn’t. Exchange drawings with yourpartner. Try to figure out which of your partner’s pentagons is similarto the original, and explain how you decided. Verify with your partnerthat you each have correctly identified the similar pentagon.M A T E R I A L SrulerM A T E R I A L S• ruler• protractor450-517_07elMSMgr7 1/4/04 9:17 AM Page 463
17. 17. On Your Own Exercises1. Look at the triangles below, but make no measurements.a. Just by looking, guess whether the triangles are congruent.b. Check your guess by finding a way to determine whether the tri-angles are congruent. Are they congruent? How do you know?2. Look at the triangles below, but make no measurements.a. Just by looking, guess whether the triangles are congruent.b. Check your guess by finding a way to determine whether the tri-angles are congruent. Are they congruent? How do you know?3. Examine these rectangles.a. Just by looking, guess which rectangle is congruent toRectangle A.b. Find a way to determine whether your selection is correct. Whichrectangle is congruent to Rectangle A? How do you know?ABCD&PracticeApply464 C H A P T E R 7 Similarity impactmath.com/self_check_quiz450-517_07elMSMgr7 1/4/04 9:17 AM Page 464
18. 18. L E S S O N 7 . 1 Are They the Same? 4654. Examine the figures below.a. Just by looking, guess which figure is congruent to Figure W.b. Find a way to determine whether your selection is correct. Whichfigure is congruent to Figure W? How do you know?5. Rectangle R is 4.5 cm by 15 cm.a. Draw and label a rectangle with sides one-third as long as thoseof Rectangle R.b. Draw and label a rectangle with sides 3 cm shorter than those ofRectangle R.c. Which of your rectangles is similar to Rectangle R?6. In Investigation 2, you explored two ways to modify rectangles andtriangles. One method produces similar figures; the other does not.In this exercise, you will examine whether either of the methods willproduce a similar figure when the original is a square.a. Draw a square that is 6 cm on a side. This is your original square.b. Make a new square with sides one-third as long as the sides ofyour original square.c. Make a new square with sides 3 cm shorter than those of youroriginal square.d. Which of the methods in Parts b and c creates a square that issimilar to your original? Explain.WXY Z4.5 cm15 cmR450-517_07elMSMgr7 1/6/04 8:28 AM Page 465
19. 19. 466 C H A P T E R 7 SimilarityDecide whether the ratios in each pair are equivalent. Explain how youdecided.7. 1:3 and 9:11 8. ᎏ12ᎏ and ᎏ23ᎏ9. 3:4 and 6:8 10. a:b and 2a:2bName two ratios that are equivalent to each given ratio.11. 2:3 12. ᎏ160ᎏ 13. 50:50Exercises 14 and 15 show a pair of similar figures. Identify all pairs ofcorresponding sides and angles.14. 15.16. Examine these triangles.a. Just by looking, guess which triangle is similar to Triangle A.b. Make some measurements to help determine whether your selec-tion is correct. Which triangle is similar to Triangle A?17. Examine these quadrilaterals.a. Just by looking, guess which is similar to Quadrilateral Z.b. Make some measurements to help determine whether your selec-tion is correct. Which quadrilateral is similar to Quadrilateral Z?WXYZABC DTHYLPOMAMSNRQLOf the approximately270 million people inthe United States, anestimated 26 millionwere born in anothercountry—a ratio of26:270, or about 1 in 10.factsJustt h e450-517_07elMSMgr7 1/4/04 9:18 AM Page 466
20. 20. L E S S O N 7 . 1 Are They the Same? 46718. In Problem Set C, you found that calculating area is not a good wayto test whether rectangles are congruent.a. Name types of figures for which calculating area is a good testfor congruence. That is, if you have two figures of that type andyou know they have the same area, you also know they must becongruent.b. Devise your own test of congruence for rectangles that will work.For each pair of figures, explain what you would measure to test forcongruence and what you would look for in your measurements.19. two circles 20. two equilateral triangles21. One way to determine whether two-dimensional figures are con-gruent is to lay them on top of each other. This test will not work,however, with three-dimensional figures.a. How could you determine whether two cereal boxes are congruent?b. How could you determine whether two cylindrical soup cans arecongruent?22. Challenge The word bisect means to divide into two equal parts.The steps below show how to bisect ЄJKL using a compass anda straightedge.Step 1 Place the compass at point K and draw an arc thatintersects both sides of the angle. Label the intersectionsX and Y.Steps 2–3 With the compass at point X, draw an arc in the interiorof ЄJKL. Using this setting, place the compass at pointY and draw another arc.Step 4 Label the intersection of these arcs H. Then draw KH៮៬.KH៮៬ is the bisector of ЄJKL.a. Describe what is true about ЄJKH and ЄHKL.b. Draw several angles and then bisect them using the steps above.JXHY LKStep 4JXLKYSteps 2–3JXY LKStep 1&ConnectExtendIf two pentagonsare similar, are theycongruent? Explainwhy or why not.If two pentagonsare congruent,are they similar?Explain why orwhy not.ownIn y o u rwords450-517_07elMSMgr7 1/4/04 9:18 AM Page 467
21. 21. 468 C H A P T E R 7 Similarity23. Maps are designed to be similar to the layout of a city’s streets. Thismap shows a section of London.a. The scale of the map is given at the right. How manyinches on the map are the same as 1,000 ft in London?Measure to the nearest ᎏ116ᎏ inch.b. What is the distance on the map along Oxford St. betweenHolles St. and Newman St.?c. What is the real distance (in feet) along Oxford St. betweenHolles St. and Newman St.?d. What is the distance on the map along New Bond St. betweenBruton Pl. and Piccadilly?e. What is the real distance along New Bond St. between Bruton Pl.and Piccadilly?24. You have two polygons that you know are similar.a. What would you measure to determine whether the two similarpolygons are also congruent?b. What would you need to know about your measurements to besure the polygons are congruent? Explain.1,000 ft450-517_07elMSMgr7 1/4/04 9:18 AM Page 468
22. 22. L E S S O N 7 . 1 Are They the Same? 46925. Preview Mariko proposed this conjecture: “If you are given twosimilar rectangles with side lengths that share a common ratio of1 to 2, the ratio of the areas is also 1 to 2. That is, the area of thelarger rectangle is twice the area of the smaller rectangle.”Is Mariko correct? If she is, explain how you know. If she isn’t, givea counterexample for which the conjecture isn’t true.26. In Investigation 1, you examined rules for testing two figures todetermine whether they are congruent. For each pair of figures inthis exercise, describe a test you could use to tell whether theyare similar.a. two circlesb. two cubesc. two cylinders27. In Investigation 4, you discovered that similar polygons have corre-sponding sides that share a common ratio and corresponding anglesthat are congruent. For some special polygons, though, you can findeasier tests for similarity. For each pair of special polygons below,find a shortcut for testing whether they are similar.a. two rectanglesb. two squaresEvaluate each expression.28. Ϫ3 ؒ 20 29. Ϫ4 ؒ Ϫ5.5 30. Ϫ2 ؒ Ϫ5 ؒ Ϫ831. ᎏ18ᎏ Ϭ ᎏ38ᎏ 32. ᎏ161ᎏ Ϭ ᎏ121ᎏ 33. ᎏ47ᎏ Ϭ 234. 5ᎏ116ᎏ Ϫ 1ᎏ18ᎏ 35. ᎏ234ᎏ ϩ ᎏ23ᎏ 36. ᎏ341ᎏ Ϫ ᎏ652ᎏExpress each answer in scientific notation. Before adding or subtracting,be sure to change one of the numbers so that both have the same exponent.37. 6 ϫ 108ϩ 3 ϫ 10738. 3.6 ϫ 104Ϫ 4.5 ϫ 10339. Arnaldo wrote 5b ϩ 6 to represent the total number of blocks in5 bags plus 6 extra blocks. If he has 66 blocks altogether, howmany blocks are in each bag?RememberRatios can be writtenin several ways:• one to two• 1 to 2• 1:2• ᎏ21ᎏReviewMixed450-517_07elMSMgr7 1/4/04 9:18 AM Page 469
23. 23. 470 C H A P T E R 7 SimilarityAlgebra Simplify each expression.40. 5p ϩ (6p Ϫ 12)41. 2x Ϫ (3x ϩ 1) ϩ (2x ϩ 8)42. (4x ϩ 1) Ϫ (3 Ϫ 6x) ϩ 243. 3g ϩ 2(h Ϫ 1) Ϫ (4g Ϫ 6)44. (20a ϩ 60b) Ϫ 3(a Ϫ 2b) ϩ 545. 2d(d Ϫ 1) Ϫ 3d(2 Ϫ 4d)Geometry Use the Pythagorean Theorem or the distance formula to findthe length of each segment.46. (6, 0) to (0, 8)47. (3, Ϫ1) to (Ϫ2, 5)48. (8, 3) to (3, 8)49. (Ϫ2, Ϫ3) to (Ϫ1.5, Ϫ4)50. Look at the pattern of squares.a. How many squares do you add to go from Stage s to Stage s ϩ 1?b. How many squares are needed for Stage 5? Stage 7?c. How are the new squares placed?d. How many squares in all are at each stage shown?e. What stage uses 99 squares?f. Write an expression for the number of squares in Stage s.Stage 2 Stage 3 Stage 4Stage 1450-517_07elMSMgr7 1/4/04 9:19 AM Page 470
24. 24. L E S S O N 7 . 2 Polygon Similarity and Congruence 471How often do you see polygons? Traffic signs, books and posters, boxes,doors and windows—they all have polygonal shapes.The most common polygon is probably the rectangle, but triangles are thesimplest polygons because they have the least possible number of sides.Two-sided figures like those below don’t close, so they aren’t polygons.Every polygon can be divided into triangles. Because of this fact, yourknowledge of triangles can help you study other polygons.ExploreUse linkage strips to form an equilateral triangle.To make sure the sides are the same length, start at one vertex andcount the gaps between the holes until you reach the next vertex.Each gap is 1 unit. All three sides of your triangle should have thesame number of units.Measure each angle of your triangle. (It may help to trace inside thefigure on a sheet of paper first.)Compare your triangle to the others made in your class. Are all thetriangles congruent? Are they all similar? How do you know?Polygon Similarityand CongruenceM A T E R I A L S• linkage strips andfasteners• protractor450-517_07elMSMgr7 1/6/04 7:32 AM Page 471
26. 26. Investigation&L E S S O N 7 . 2 Polygon Similarity and Congruence 473ShareSummarize1. You can test whether any two polygons are congruent by measur-ing the sides and angles of both figures. Make a conjecture aboutwhat might be an easier test if the polygons are triangles.2. Give an example to show your test won’t work for other polygons.22 And More SidesYou know that you can test whether two polygons are congruent bymeasuring all their sides and angles. For triangles, though, you need tomeasure only their sides. This simpler test is called the side-side-sidecongruence test, or SSS for short. Congruent figures must have corre-sponding sides of equal length. For similar figures, rather than havingequal length, corresponding side lengths must share a common ratio.When you compared figures for similarity, you could have measured all thesides and angles as you did for congruence. In this investigation, you willexplore whether there is a test for similarity like the SSS congruence test.Problem Set CWith your partner, you will make triangles and quadrilaterals. For eachproblem, each of you should create one of the figures. See if you and yourpartner can create figures that are definitely not similar.Trace the insides of your figures. If you can create nonsimilar figures,explain how you know they are not similar. If you can create only similarfigures, tell how you know they are similar.1. Make one triangle with sides of length 3, 2, and 2 units, and onewith sides of length 6, 4, and 4 units.2. Make one quadrilateral with sides of length 8, 6, 8, and 6 units, andone with sides of length 4, 3, 4, and 3 units. Keep the sides in theorder given.3. Make one triangle with all sides 2 units long, and one with all sides 6units long.4. Make one quadrilateral with all sides 2 units long, and one with allsides 6 units long.5. Make one triangle with sides of length 3, 4, and 5 units, and onewith sides of length 4, 5, and 6 units.M A T E R I A L Slinkage strips andfasteners450-517_07elMSMgr7 1/6/04 11:13 AM Page 473
27. 27. &&474 C H A P T E R 7 SimilarityThink DiscussFor each pair of figures below, can you tell whether the two figuresare similar by knowing just the lengths of their sides? If so, make aconjecture of a way to test for similarity. If not, give a counterexam-ple showing that this information is not enough.1. two triangles 2. two quadrilateralsProblem Set DSome of the triangles below are similar to each other. Work with a partnerto group all the triangles so that those in each group are similar.1. 2. 3.4. 6.10. 11. 12.ShareSummarizeWhen it comes to congruence and similarity, how are triangles differentfrom other polygons?RememberUntil it is proven, youcan’t be sure a conjec-ture is true.M A T E R I A L S• metric ruler• protractorSailors have long usedimaginary triangles—formed by their vessel,the horizon, and celes-tial bodies—to deter-mine their position onthe seas.factsJustt h e5.7.8. 9.450-517_07elMSMgr7 1/6/04 7:32 AM Page 474
28. 28. &InvestigationL E S S O N 7 . 2 Polygon Similarity and Congruence 47533 Angles, Angles, AnglesAs you saw in Investigations 1 and 2, you can simply measure the sidesof two triangles to test whether they are congruent or similar. With otherpolygons, you need to know about their angles as well. What if the onlyinformation you have about two polygons is the measures of their angles?Problem Set EWork with a partner to draw a figure that fits each description. Write theproblem numbers inside the figures.1. a triangle with angles 30°, 60°, and 90°2. a quadrilateral with all anglesmeasuring 90°3. a triangle with two 60° angles4. a quadrilateral with three 80° angles5. a triangle with two 45° angles6. a triangle with one 110° angle and one25° angle7. a triangle with one 90° angle8. Compare your figures to those made by others in your class. Forwhich problems were all of the figures similar?ShareSummarize1. Can you tell whether two triangles are similar by knowing justtheir angle measures? If so, make a conjecture of a test you canuse. If not, give a counterexample showing that this informationis not enough.2. Will your test work on quadrilaterals? If so, explain. If not, givea counterexample.3. Can you tell whether two triangles are congruent by knowing justtheir angle measures? If so, make a conjecture of a test you canuse. If not, give a counterexample.M A T E R I A L S• ruler• protractor450-517_07elMSMgr7 1/4/04 9:20 AM Page 475
29. 29. LabLabInvestigation476 C H A P T E R 7 SimilarityBuilding TowersArchitects need to know a lot about geometry and the properties of fig-ures. Some of what you have learned in this lesson may help you builda model as an architect would.The ChallengeWork with your group for 10 minutes to build the tallest freestandingstructure you can. A freestanding structure is one that does not leanagainst anything; it stands on its own. After 10 minutes, measure yourstructure from the table to its highest point.Evaluating Your Work1. How tall is your structure?2. Look around the room at the different structures. What strategies didthe creators of the tallest structures use?3. What are the common features of the structures that have troublestanding up? What are the common features of the structures thatare stronger?The SSS congruence test you learned about works only for trianglesbecause triangles are the only polygons that are rigid. That is, if you builda triangle with three unbendable sides, you can’t press on the sides or theangles of the triangle to make a different shape. However, you can changethe shapes of other polygons in an infinite number of ways by pressing onthe sides. The vertices act like hinges.Pushing on the sides of a triangle will not change thetriangle’s shape, but pushing on the sides of otherpolygons will change their shapes.4. Does the idea of rigidity help explain which structures seemstronger? If so, explain.5. If you had the chance to create another structure, what buildingstrategies would you try?M A T E R I A L S• toothpicks• mini-marshmallows(or otherconnectors)• rulers450-517_07elMSMgr7 1/6/04 7:33 AM Page 476
30. 30. L E S S O N 7 . 2 Polygon Similarity and Congruence 477Try It AgainTry your building strategies with another set of materials for another10 minutes. Your goal this time is to build a freestanding structure tallerthan your first attempt.6. How tall is your second structure?7. What strategies did you use this time?What Did You Learn?8. Why do you think buildings have triangular supports in their walls?9. Which of these structures is likely to remain standing the longest?Why?a. b. c.10. The foreman of a crew that wasknocking down a building consideredtelling the crew to first take out thefront and rear walls, leaving a structurelike this. Is this a wise decision? Why?450-517_07elMSMgr7 1/6/04 7:33 AM Page 477
31. 31. On Your Own ExercisesIn Exercises 1–3, decide whether the figures described must be congruent,could be congruent, or are definitely not congruent.1. two quadrilaterals with all sides 4 cm long2. two triangles with all sides 4 cm long3. two squares with all sides 4 cm longThe triangles in each pair below are congruent. Find the values ofthe variables.4.5.6.7. The side lengths of six triangles are given. Tell which triangles aresimilar to a triangle with sides of length 2 cm, 4 cm, and 5 cm.a. 1 in., 7 in., 4 in.b. 5 cm, 4 cm, 2 cmc. 8 in., 4 in., 10 in.d. 400 cm, 500 cm, 200 cme. 20 ft, 30 ft, 40 ftf. 1 cm, 2 cm, 2.5 cm8 cm9 cm9 cm2 cmmnacb1 cm1 cm1 cm4 cm3 cmx5 cm4 cmy&PracticeApply478 C H A P T E R 7 Similarity impactmath.com/self_check_quiz450-517_07elMSMgr7 1/4/04 9:21 AM Page 478
32. 32. L E S S O N 7 . 2 Polygon Similarity and Congruence 479In Exercises 8–17, decide whether the figures described must be similar,could be similar, or are definitely not similar.8. two squares9. two rectangles10. two triangles11. a square with all sides 2 cm and a rectangle with length 2 cm andwidth 1 cm12. a triangle with sides of length 2 cm, 3 cm, and 4 cm and a trianglewith sides of length 2 ft, 3 ft, and 4 ft13. a triangle with all sides 2 cm long and another triangle with all sides2 cm long14. two quadrilaterals with all right angles15. two triangles, each with a right angle16. two triangles with three 60° angles17. two triangles with two 45° angles18. Sally thinks that triangles with two corresponding sides of equallengths and an equal angle must be congruent. To test her conjecture,first draw as many triangles as you can that meet the descriptions inParts a–c.a. Two sides have lengths 4 and 7; the angle between them is 115°.b. Two sides have lengths 5 and 8; the angle between them is 35°.c. Two sides have lengths 4 and 5; one of the angles not between thetwo sides is 45°.d. Do you agree with Sally’s conjecture? That is, if you know thattwo corresponding sides and a corresponding angle of two trian-gles are congruent, must the triangles be congruent? Explain.19. Suppose you have two circles. Decide whether they must be similar,could be similar, or are definitely not similar.&ConnectExtend450-517_07elMSMgr7 1/4/04 9:22 AM Page 479
34. 34. L E S S O N 7 . 2 Polygon Similarity and Congruence 481a. You now have four small triangles. How do they compare tothe large triangle you began with? (Think about similarity andcongruence.)b. How do the four small triangles compare to each other?Now, for each triangle that is not shaded, find the midpoints of thesides. In each of these triangles, connect the midpoints. Then shadethe new upside-down triangles.c. You now have 13 triangles inside the original large one. Are allthe triangles similar? Are any of them congruent?Evaluate without using a calculator.23. 49.073 Ϫ 56.2 24. 0.98 Ϫ 2.71 25. 400.06 Ϫ 2.226. ᎏ37ᎏ ϩ ᎏ49ᎏ 27. ᎏ23ᎏ Ϫ ᎏ17ᎏ 28. ᎏ3325ᎏ ϩ ᎏ730ᎏFactor each expression.29. 3p ϩ 3 30. ᎏa7ᎏ Ϫ ᎏ17ᎏ 31. 4a2b Ϫ 16a3Supply each missing exponent.32. 6 ϫ 10?ϭ 600 33. 0.2 ϫ 10?ϭ 0.002 34. 1.7 ϫ 10?ϭ 1735. Suppose you use a photocopying machine to enlarge a picture.a. Your picture is 10 cm wide and you enlarge it 110%. How wide isthe copy of the picture?b. You use the same setting to enlarge your copy. How wide is thefinal picture?36. Suppose you use a photocopying machine to reduce a picture.a. Your picture is 10 cm wide and you reduce it to 85% of itsoriginal size. How wide is the copy of the picture?b. You use the same setting to reduce your copy. How wide is thefinal picture?ReviewMixed450-517_07elMSMgr7 1/4/04 9:22 AM Page 481
35. 35. Investigation&482 C H A P T E R 7 SimilarityYou have learned several tests for identifying similar figures, and youknow a few ways to create similar figures. How do you think the areas andperimeters of similar figures compare? In this lesson, you will find out.Think DiscussYou can combine nine copies of anequilateral triangle to form a larger triangle.Is the large triangle similar to the smalltriangle? How do you know?If the area of the small triangle is 1 squareunit, what is the area of the large triangle?Suppose you wrap a string around the perimeter of the small triangle.How many times longer must a string be if you want to wrap itaround the perimeter of the large triangle?11 Dilating Trianglesand RectanglesYou have seen that if two polygons are similar, the lengths of their corre-sponding sides share a common ratio. That means you can multiply theside lengths of one figure by some number to get the side lengths of theother figure.Given similar figures A and B, the scale factor from Figure A to Figure Bis the number by which you multiply the side lengths of Figure A to getthe side lengths of Figure B. The scale factor from the small triangle tothe large triangle shown above is 3. Figure B is said to be a dilation ofFigure A. Dilating a figure creates another figure that is similar, but notnecessarily congruent, to the original.Area and Perimeterof Similar FiguresV O C A B U L A R Yscale factordilation450-517_07elMSMgr7 6/7/04 5:07 PM Page 482
36. 36. L E S S O N 7 . 3 Area and Perimeter of Similar Figures 483Problem Set AThe sides of this small triangle are 1 unit long. You will buildlarger equilateral triangles from copies of this small triangle.1. Trace and cut out several copies of the small triangle.a. Use your triangles to build larger triangles with the side lengthslisted in the table. Make a sketch of each large triangle you build.b. Copy and complete the table.Side Length Number of Smallof Large Triangles in Scale FactorTriangle Large Triangle (small to large)12342. Make a new table with three columns. In the first column, list thescale factors you found in Problem 1. You will complete the othertwo columns in Parts a and b.a. First, find the perimeter of each large triangle you cre-ated. In the second column of your table, recordthe ratios of the perimeter of the small trian-gle to the perimeter of the large triangle.Write each ratio using the smallestpossible whole numbers. For example, the tri-angle at right has a perimeter of 9 units, so the ratioof the perimeters is 3:9, which is equivalent to 1:3.b. Now find the area of each large triangle, using the area of thesmall triangle as your unit. For example, the area of the largetriangle above is 9 small triangles. In the third column of yourtable, give the ratios of the areas.3. How are the perimeters of the small and the large triangles related?4. How are the areas of the small and the large triangles related?5. You’ve worked with dilating the small triangle to make the largetriangle. In fact, each pair of similar figures with different sizes hastwo scale factors associated with it. Imagine dilating each of thelarge triangles down to the size of the small triangle.a. For each, what would the scale factor from large to small be?b. How is the large-to-small scale factor for a pair related to thesmall-to-large scale factor?M A T E R I A L S• scissors• straightedge450-517_07elMSMgr7 6/7/04 5:08 PM Page 483
37. 37. &484 C H A P T E R 7 SimilarityProblem Set BEach large rectangle below is divided into smaller rectangles that arecongruent to each other and similar to the large rectangle.For Problems 1–5, find the scale factor from the small rectangle to thelarge rectangle. Then calculate the perimeter and the area of each sizerectangle. Record your answers in two tables, with these heads:Problem Scale Factor Perimeter of Perimeter ofNumber (small to large) Small Rectangle Large RectangleProblem Scale Factor Area of Area ofNumber (small to large) Small Rectangle Large Rectangle1. 2. 3.4. 5.ShareSummarize1. A rectangle with perimeter p and area A is enlarged by a factorof f. Make a conjecture about how to calculate the perimeter andthe area of the new rectangle.2. A triangle with perimeter p and area A is enlarged by a factorof f. Make a conjecture about the perimeter and the area of thenew triangle.3. Use your observations of similar rectangles and similar trianglesto make a conjecture about how you can calculate the perimetersand the areas of two similar polygons.3 units2 units2 units1 unit1 unit3 units2 units2 units1 unit1 unit450-517_07elMSMgr7 1/4/04 9:23 AM Page 484
38. 38. Investigation 22 Dilation and FormulasThe box below contains possible conjectures for the relationship betweenthe perimeters and between the areas of similar polygons.For two similar polygons A and B such that the scale factor fromPolygon A to Polygon B is r,• the perimeter of Polygon B is r times the perimeter of Polygon A.• the area of Polygon B is r2times the area of Polygon A.Your investigation with whole-number scale factors supports these con-jectures—but you can’t try every possible scale factor to verify them.However, you can use the perimeter and area formulas for a polygon toprove that the conjectures are true for any scale factor and that polygon.Malik thought of a way to verify the perimeter conjecture for all rectangles and a scale factor of 3.When I dilate arectangle by a wholenumber, the perimeteris dilated by the samenumber. But what if Idont know anythingabout the rectangle?For length L and width W,the rectangles perimeteris L + W + L + W. I justadd the side lengths.If its dilated by 3, the newlength is 3L and the newwidth is 3W – so the newperimeter is3L + 3W + 3L + 3W. Thedistributive property lets mefactor out a 3, giving3(L + W + L + W), which is3 times the original perimeter.WL3W3LE X A M P L EE X A M P L EL E S S O N 7 . 3 Area and Perimeter of Similar Figures 485450-517_07elMSMgr7 6/7/04 5:11 PM Page 485
39. 39. Problem Set C1. Following the steps below, you will use anexplanation like Malik’s to show that if you dilatethis square by a factor of ᎏ12ᎏ, the perimeter of thenew square will be ᎏ12ᎏ the original perimeter.a. Malik first wrote the perimeter of the rectangle as a sum of thefour side lengths. Write the sum that gives the perimeter of theoriginal square.b. Dilate the square by a factor of ᎏ12ᎏ, and draw the new square. Howlong is each side, in terms of s?c. Write the sum that gives the perimeter of the new square.d. Is the new perimeter half the original perimeter? Explain how youknow from the expressions you wrote in Parts a and c.2. Following the steps below, you will showthat if you scale this triangle by 100, the newperimeter will be 100 times the originalperimeter.a. Write an expression for the triangle’sperimeter.b. If you dilate the triangle by a factor of 100, how long will eachside be?c. Write an expression for the perimeter of the dilated triangle.d. Is the new perimeter 100 times the original? Explain.3. Prove It! Suppose a pentagon has sides of length a, b, c, d, and e.a. Show that if you dilate the pentagon by 0.323, the new perimeterwill be 0.323 times the original perimeter.b. Show that if you dilate the pentagon by a factorof r, the new perimeter will be r times theoriginal perimeter.c. Explain why a similar argument wouldwork for a polygon with any numberof sides, not just for five-sidedpolygons.bcassM A T E R I A L SrulerModel trains such as this oneare built to specific scales. Acommon scale is 1:87, whichmeans that 87 feet of actualtrack would be represented by1 foot of track on the model.486 C H A P T E R 7 Similarity450-517_07elMSMgr7 6/7/04 5:12 PM Page 486
40. 40. L E S S O N 7 . 3 Area and Perimeter of Similar Figures 487Now you will consider what happens to the area of a dilated figure.A square with side length s has an area of s2.If you dilate the square by 3, the new sides have length 3s.The area of the new square is (3s)2ϭ 9s2, which is nine times theoriginal area.In general, if you dilate an s ϫ s square by a factor of r, the newsquare has sides of length rs.The area of the new square is (rs)2ϭ r2s2, which is r2times theoriginal area.rsrs3s3sssE X A M P L EE X A M P L EProblem Set D1. In this problem, you will show that theareas of similar parallelograms are relatedby the square of the scale factor betweenthem. This parallelogram has length l andheight h.a. What is the parallelogram’s area?b. If you dilate the parallelogram by a factor of 2, what happens tothe height? Explain your thinking.c. If you dilate the parallelogram by a factor of 2, what are the newlength and height?d. What is the area of the dilated parallelogram?lh450-517_07elMSMgr7 6/7/04 5:13 PM Page 487
41. 41. &488 C H A P T E R 7 Similaritye. If you dilate the original parallelogram by a factor of ᎏ13ᎏ, what arethe new length, height, and area?f. If you dilate the original parallelogram by a factor of r, what arethe new length and height?g. Prove It! What is the area of the dilated parallelogram in Part f?How does it relate to the area of the original parallelogram?2. In this problem, you will show that if youdilate any triangle, the two areas are related bythe square of the scale factor. Start with a trian-gle with sides x and y, base z, and height h.a. What is the triangle’s area?b. If you dilate the triangle by a factor of 3.2,what are the new base and height?c. What is the area of the dilated triangle?d. If you dilate the triangle by a factor of r, what are the new baseand height?e. Prove It! What is the area of the dilated triangle in Part e? Howdoes it relate to the area of the original triangle?ShareSummarize1. If you dilate a polygon by a factor of r, what happens to the poly-gon’s perimeter? Does it depend on r? Do you know this is truefor all polygons, or is this a conjecture?2. If you dilate a polygon by a factor of r, what happens to the poly-gon’s area? Does it depend on r? Do you know this is true for allpolygons, or is this a conjecture?zxhy450-517_07elMSMgr7 6/7/04 5:13 PM Page 488
42. 42. InvestigationL E S S O N 7 . 3 Area and Perimeter of Similar Figures 48933 Similarity in MoreComplex FiguresDo the relationships you found for similar parallelograms and triangleshold for more complex polygons? Do they hold for figures with curvedsides? You will explore these questions in this investigation.Problem Set EThe circles and the polygons in Problems 1 and 2 are similar figures. Foreach pair, do Parts a and b. You may want to record your answers in a table.a. Find the scale factor from the small figure to the large figure.b. Find the perimeters and the areas of both figures.1.2.3. How are the perimeters of two similar figures related? Do the shapesof the figures affect this relationship?4. How are the areas of two similar figures related? Do the shapes ofthe figures affect this relationship?151186464 3322513151212112RememberA circle with radius rhas area ␲r2and cir-cumference (perimeter)2␲r. A trapezoid withbases b1 and b2 andheight h has areaᎏ21ᎏ(b1 ϩ b2)h.450-517_07elMSMgr7 1/4/04 9:26 AM Page 489
43. 43. 490 C H A P T E R 7 SimilarityBy now you should have a good idea of the relationships between theareas and the perimeters of any dilated figures. To help you see that thoserelationships can be extended beyond polygons and circles, think abouthow you learned to find areas and perimeters of irregular figures.You can approximate the area of an irregular figure by placing it on a gridand counting the squares inside.If you dilate the figure along with the grid, you get something that lookslike this.You can approximate the perimeter of an irregular figure by using linesegments.When you dilate the figure, the line segments look like this.450-517_07elMSMgr7 6/7/04 5:16 PM Page 490
44. 44. &L E S S O N 7 . 3 Area and Perimeter of Similar Figures 491Problem Set F1. Write a few sentences to explain why dilating the irregular figure onthe opposite page by a factor of r dilates its area by a factor of r2.2. Write a few sentences to explain why dilating the irregular figure bya factor of r also dilates its perimeter by a factor of r.3. The figures below are similar.a. What is the scale factor from the small figure to the large figure?b. What is the area of the large figure?4. The figures below are similar.a. What is the scale factor from the small figure to the large figure?b. What is the perimeter of the small figure?ShareSummarizeThese four figures are all similar. Figure Ahas an area of 3 square units. Oneof Figures B, C, and D hasan area of 12 square units.Which figure is it? Howcan you tell?Perimeter: ? unitsArea: 4 square unitsPerimeter: 24 unitsArea: 16 square unitsPerimeter: 5 unitsArea: 1 square unitPerimeter: 15 unitsArea: ? square unitsABCDM A T E R I A L SrulerThe arms of certainstarfish, when brokenoff, will regenerate intonew starfish geneticallyidentical to the originalorganisms. This processis known as cloning.factsJustt h e450-517_07elMSMgr7 6/7/04 5:16 PM Page 491
45. 45. On Your Own ExercisesIn Exercises 1–4, a figure is divided into smaller figures that are congru-ent to each other and similar to the large figure. Do Parts a–c for eachexercise.a. Find the scale factor from the small figure to the large figure.b. Find the perimeter and area of each figure (small and large).c. Test whether the results support your conjectures from theShare & Summarize on page 488.1. 2.3.4.height = 21294height = 121318height = 11272015534&PracticeApplyRememberThe area of a trianglewith base b and heighth is ᎏ21ᎏbh. The area ofa parallelogram withbase b and height his bh.492 C H A P T E R 7 Similarity impactmath.com/self_check_quiz450-517_07elMSMgr7 1/4/04 9:27 AM Page 492
46. 46. L E S S O N 7 . 3 Area and Perimeter of Similar Figures 4935. This rectangle has sides x and y.a. What is the rectangle’s area?b. If you dilate the rectangle by a factor of 10, what is the area ofthe dilated rectangle?c. Prove It! Show that if you dilate the rectangle by a factor of r,the area of the new rectangle is r2times the area of the original.6. A triangle has perimeter 10 cm and area 4 cm2. Could you makea similar figure with perimeter 30 cm and area 12 cm2? Why orwhy not?7. A rectangle has perimeter 30 cm and area 25 cm2. Could you make asimilar figure with perimeter 6 cm and area 1 cm2? Why or why not?8. Here are two similar figures.a. What is the scale factor from the large figure to the small figure?How did you find it?b. What is the scale factor from the small figure to the large figure?c. Suppose you wrapped a string around the small figure and nowwant a piece of string to wrap around the large figure. In compari-son to the length of string needed for the small figure, how muchstring will you need for the large figure?Area = 32 square units Area = 18 square unitsxy450-517_07elMSMgr7 6/7/04 5:17 PM Page 493
47. 47. 494 C H A P T E R 7 Similarity9. Here are two similar figures.a. What is the scale factor from the large figure to the small figure?b. What is the scale factor from the small figure to the large figure?How did you find it?c. If the area of the small figure is n square meters, what is the areaof the large figure?10. A figure has perimeter 30 cm and area 50 cm2. Could you makea similar figure with perimeter 60 cm and area 100 cm2? Why orwhy not?11. The area of this triangle is 2 square units. Draw a triangle similar toit that has an area of 18 square units.12. Measurement Understanding the relationships in Investigation 1can help you with metric conversions.a. What is the scale factor from a square with a side length of 1 cen-timeter to a square with a side length of 1 meter?b. How many square centimeters fit in a square meter?c. What is the scale factor from a cube with a side length of 1 cen-timeter to a cube with a side length of 1 meter?d. How many cubic centimeters fit in a cubic meter?13. Prove It! The area of a regular hexagon with side lengths s is givenby ᎏ32ᎏ͙3ෆs2.Suppose you dilate this hexagon by a factor of f. Prove that the areaof the new hexagon is f2times the area of the original hexagon.sPerimeter: 27 m Perimeter: 2.7 m&ConnectExtendSuppose you havetwo similar figures,one with a perime-ter of twice theother. Explain whythe area of one isnot twice the areaof the other.ownIn y o u rwords450-517_07elMSMgr7 6/7/04 5:17 PM Page 494
48. 48. L E S S O N 7 . 3 Area and Perimeter of Similar Figures 49514. Architecture At left is a scale drawing of Zeitner Tower, a steel-framed 12-story building. The scale factor from the drawing to thereal building is 1:480.a. What is the height of the real Zeitner Tower?b. At right is a sketch of the first floor of thebuilding, using the same scale factor. What isthe area of the sketch? What is the area of thefirst floor of Zeitner Tower?c. Suppose you wanted to fill Zeitner Tower with popcorn. Whatvolume of popcorn would you need? Explain.15. The grid lines on this map of a lake are at 1-centimeter intervals. Usethe map’s scale to approximate the area of the real lake’s surface.16. Life Science Scientists often workwith objects that are too small to seewith the unaided eye. A microscope,in a way, creates a scale model ofsmall objects. For example, thisphotograph of a Volvox colony ismagnified 150 times.a. Measure the approximate diameterof this entire colony as magnified,in millimeters.b. What is the approximate diameter of the real Volvox colony?Beach AreaBike PathBoat LaunchScale= mile1–4Volvox, a member of theplant kingdom, is atype of algae. Thesespherical colonies con-tain 500 to 500,000cells in a single layersurrounding the fluid-filled center. Thecolonies live in waterand are able to swimby the beating of smallhairlike structures ontheir surface, calledflagella.factsJustt h e450-517_07elMSMgr7 6/7/04 5:18 PM Page 495
49. 49. 496 C H A P T E R 7 SimilarityRewrite each expression as a fraction.17. (Ϫ10)–518. (Ϫ4)Ϫ2ؒ (Ϫ4)Ϫ219. 3Ϫ6Ϭ 3Ϫ2Rewrite each expression as addition, and calculate the sum.20. Ϫ10 Ϫ Ϫ9 21. 1.5 Ϫ Ϫ0.14 22. Ϫ3.33 Ϫ Ϫ1.6823. Draw a factor tree to find all the prime factors of 128.24. Copy and complete the table.Fraction Decimal Percentᎏ12ᎏ 0.5 50%85%ᎏ45ᎏ125. Order these fractions from least to greatest.ᎏ12ᎏ Ϫᎏ12ᎏ ᎏ18ᎏ ᎏ14ᎏ Ϫᎏ13ᎏ ᎏ19ᎏ Ϫᎏ17ᎏ26. Point A is located at (5, 2).a. Point (3, 1) is located 3 units from Point A along grid lines. Nameall other points that are located 3 units from Point A along grid lines.(Be careful—some of the points may not be visible on the gridshown.)b. Point B is located at (3, 5). How many units long is the shortestpath from Point A to Point B along grid lines?c. How many different shortest paths are there from Point A toPoint B along grid lines?27. Two cylinders have the same radius. The height of one cylinder isthree times the height of the other cylinder. How do the volumes ofthe cylinders compare?765432110 2 3 4 5 6 7 8AReviewMixed450-517_07elMSMgr7 1/6/04 7:34 AM Page 496
50. 50. L E S S O N 7 . 4 Volume and Surface Area of Similar Figures 497You have dilated two-dimensional drawings. You can also dilatethree-dimensional objects. Model trains are scale models of real trains,a globe is a scale model of Earth, and some dollhouses are scale modelsof real houses.To see how scaling works in three dimensions, it helps to start with sim-ple block structures.ExploreBuild this block structure.Which of the block structures below has edges that are twice as longas those of your structure? You may want to use your blocks to helpyou answer the question.Does the structure you identified have twice the volume of yourstructure?AD EB CVolume andSurface Area ofSimilar FiguresM A T E R I A L Scubes450-517_07elMSMgr7 6/7/04 5:19 PM Page 497
51. 51. &Investigation498 C H A P T E R 7 Similarity11 Dilating Block StructuresAs with two-dimensional figures, the scale factor between three-dimensional figures is the number by which you multiply the lengthsof the original figure to get the corresponding lengths of the new figure.The figure you identified in the Explore activity is similar to the originalstructure, dilated by a factor of 2.Problem Set AFor each structure, work with a partner to build a new structure, using ascale factor of 2. Draw the top-count view of each structure you build.(Remember: The top-count view shows a top view of the structure andthe number of blocks in each stack.)1. 2. 3.Problem Set BFor each structure, work with a partner to build a new, similar structure,scaled by the given factor. Draw a top-count view of each structure youbuild.1. scale factor 3 2. scale factor 33. scale factor ᎏ12ᎏ 4. scale factor ᎏ23ᎏShareSummarizeWrite a letter to a friend explaining the steps you took to solve Problem 2of Problem Set A.M A T E R I A L S• cubes• graph paperM A T E R I A L S• cubes• graph paper450-517_07elMSMgr7 6/7/04 5:19 PM Page 498
52. 52. InvestigationL E S S O N 7 . 4 Volume and Surface Area of Similar Figures 49922 Volume of SimilarBlock StructuresIn Lesson 7.3, you found that if you create a polygon that is dilated by afactor of c from an original polygon, the perimeter of the new polygonwill be c times the original perimeter. You also found that the area will bec2times the original area. Now you will dilate three-dimensional objectsand compare the volumes of the dilated objects to the original volumes.Problem Set CWork in a group on these problems.1. For each block structure, find the volume of the original. Then createa structure that has edges twice as long as those of the original struc-ture, and find the new volume. Record your results in a table like theone shown.a. b. c.Volume of Volume ofPart Original Structure Scaled Structureabc2. You dilated each structure in Problem 1 by a factor of 2. For eachstructure, how does the volume of the dilated structure appear to berelated to the volume of the original structure?M A T E R I A L ScubesRememberA single block has avolume of 1 cubic unit.450-517_07elMSMgr7 6/7/04 5:19 PM Page 499
53. 53. &500 C H A P T E R 7 SimilarityProblem Set DNow you will explore what happens to volume for several scale factors.Your original structure for these problems will be a single block, whichhas a volume of 1 cubic unit.Working with your group, complete the table below by following theseinstructions:• If you are given the scale factor, try to build a new block structuresimilar to the original structure and scaled by that factor. Find thenew volume.• If you are given the volume, try to build a block structure similar tothe original structure and with that volume. Find the scale factor.For some entries, you won’t be able to build the structure, but you will beable to reason about what the volume would be if you could build it.Scale Volume ofFactor Dilated Structure227110ᎏ18ᎏᎏ15ᎏrShareSummarizeMake a conjecture describing how the volume changes when you dilatea block structure. Be sure to say how the change in volume depends onthe scale factor.M A T E R I A L Scubes450-517_07elMSMgr7 6/7/04 5:20 PM Page 500
54. 54. InvestigationL E S S O N 7 . 4 Volume and Surface Area of Similar Figures 501M A T E R I A L S• metric ruler• scissors• compass• tapeRememberA net is a flat objectthat can be folded toform a closed, three-dimensional solid.33 Surface Area ofSimilar ObjectsIn Investigation 2, you explored the relationship between the volumes oftwo similar objects. Another measurement for three-dimensional objects issurface area.Nets allow you to use only two dimensions to show all the surfaces of athree-dimensional object at once. Nets make it easy to explore the rela-tionship between the surface areas of two similar objects.Problem Set E1. Here is a net for a square prism.a. Carefully draw and label a net similar to this one, using a scalefactor of 2.b. Trace the net above. Cut out both nets, and fold them to formsolids. Are the two prisms similar?c. Compare the surface areas ofyour two prisms. You mightfind it easier to work withthe unfolded nets. Howmany times the surface areaof the scaled prism is thesurface area of the originalprism? Show how you foundyour answer.2 cm2 cm3 cm450-517_07elMSMgr7 1/4/04 9:29 AM Page 501
55. 55. 502 C H A P T E R 7 SimilarityProblems 2 and 3 show a net for a prism and a cylinder. For each prob-lem, do Parts a–c.a. Draw and label a net similar to the given net, using the givenscale factor.b. Find the surface areas of the original object and the scaled object.c. Compare the surface areas. The new surface area is how manytimes the original surface area?2. scale factor 33. scale factor 44. How does the surface area of a scaled object appear to be related tothe surface area of the original object? Be sure to mention scalefactors in your answer.In Lesson 7.3, you saw that the relationships you found between peri-meters of scaled polygons and areas of scaled polygons are true for allfigures, not just polygons. It’s also true that the volumes and surface areasof all similar objects have the same relationships as the ones you’ve prob-ably observed.That is, if three-dimensional Figures A and B are similar and the scalefactor from Figure A to Figure B is r, then• the volume of Figure B is r3times the volume of Figure A.• the surface area of Figure B is r2times the surface area of Figure A.5 cm1 cm3 cm4 cm0.5 cm1 cmRememberThe surface area of acylinder with radius rand height h is2␲r2ϩ 2␲rh (thearea of the top andbottom plus the areaof the sides).450-517_07elMSMgr7 1/4/04 9:30 AM Page 502
56. 56. L E S S O N 7 . 4 Volume and Surface Area of Similar Figures 503Problem Set F1. The figures below are similar.Surface area: 8 square units Surface area: 200 square unitsVolume: 2 cubic units Volume: ? cubic unitsa. Find the scale factor of the small figure to the large figure.b. Find the volume of the large figure.2. The figures below are similar.a. Find the scale factor of the small figure to the large figure.b. Find the surface area of the small figure.Surface area: ? square units Surface area: 450 square unitsVolume: 20 cubic units Volume: 540 cubic unitsM A T E R I A L Sruler450-517_07elMSMgr7 1/6/04 8:42 AM Page 503
57. 57. &Investigation&504 C H A P T E R 7 Similarity3. The figures below are similar.Surface area (hull): 40 square units Surface area (hull): ? square unitsVolume (hull): 10 cubic units Volume (hull): ? cubic unitsa. The small boat’s hull is half the length of the large boat’s hull.Find the scale factor of the small figure to the large figure.b. Find the surface area and volume of the large boat’s hull.ShareSummarizeSuppose Figures A and B are similar figures such that the scale factorfrom Figure A to Figure B is n.1. If the surface area of Figure A is s, what is the surface area ofFigure B?2. If the volume of Figure A is v, what is the volume of Figure B?44 GiantsIn Jonathan Swift’s 1726 book Gulliver’s Travels, Gulliver visits a placecalled Brobdingnag. There he meets people who are 12 times his size inevery dimension. Could the Brobdingnagians really have existed? If thebody of a giant were similar to ours, would it be able to support its ownweight?Think DiscussIn Chapter 2, you used blocks to model human beings so you couldthink about people’s surface areas more easily. If you use a singleblock to model Gulliver, how many blocks would you need to modela giant 12 times the size in every dimension?If Gulliver weighed 175 lb, how much would the giant weigh (basedon his volume)?The hull is the mainbody of a boat, notincluding the masts,sails, or rigging.factsJustt h e450-517_07elMSMgr7 1/6/04 7:35 AM Page 504
58. 58. &L E S S O N 7 . 4 Volume and Surface Area of Similar Figures 505Problem Set GYour bones must be able to support more than your own weight. If theydidn’t, you couldn’t carry a heavy object, gain weight, or land from ajump without breaking your bones. The maximum weight your bonescan support depends on their thickness, or cross-sectional area.1. The sketch shows a cross section ofGulliver’s femur (thigh bone), whichis very close to a circle. What is theapproximate area of the circular crosssection?2. For mammals, the weight a bone cansupport is related to the area of the bone’scross section. If its cross-sectional areais n in.2, a femur can support 1,563n lb.How much weight can Gulliver’s femursupport?3. Giants in Brobdingnag are 12 times thesize of Gulliver in every dimension. Whatis the area of the circular cross section ofa giant’s femur?4. Assume that the giant’s bones have the same strength as humanbones. Using the fact that a bone with cross-sectional area n in.2cansupport 1,563n lb, find the maximum weight the giant’s femur cansupport.5. When you walk, one leg must support all your weight as you take astep. Could the giant’s leg bone support his weight? Explain howyou know.6. For a leg bone to support the giant’s weight (as a maximum), howmuch cross-sectional area must it have? What would its radius be?Explain.ShareSummarize1. Explain why there cannot really be giants the same shape as peoplewith bones the same strength as human bones. What do you thinkis the maximum size a person could grow to and still stand up?2. Giants that can support their own weight cannot be similar tohumans. What would giants who can support their weight haveto look like? Explain.0.6 in.Jonathan Swift’smasterpiece—the fulltitle of which is Travelsinto Several RemoteNations of the World,by Lemuel Gulliver—iswritten in the form of ajournal kept by Gulliver,a ship’s physician.factsJustt h e450-517_07elMSMgr7 1/6/04 11:13 AM Page 505
59. 59. On Your Own Exercises1. Bryn dilated Structure A by some factor to get Structure B. Whatscale factor did Bryn use? How do you know?Determine whether the block structures in each pair below are similar. Ifthey are, find the scale factor. If they aren’t, explain how you know.2. 3.4. Suppose you dilate this block structure by a factor of ᎏ12ᎏ. What wouldthe volume of the new structure be? Explain.Structure A Structure B&PracticeApply506 C H A P T E R 7 Similarity impactmath.com/self_check_quiz450-517_07elMSMgr7 6/7/04 5:23 PM Page 506
60. 60. L E S S O N 7 . 4 Volume and Surface Area of Similar Figures 5075. Using cardboard, Enrique made a box 30 cm long, 30 cm wide, and12 cm deep. The box has no top. Enrique then wanted to make a newbox, similar to the first one, that would hold exactly twice as much.a. What is the volume of Enrique’s original box?b. If its sides were twice as long as those of the original box, howmuch would the new box hold?c. Would the sides of the new box that holds twice as much be twiceas long as the sides of the original box?d. Challenge Find the approximate dimensions of a box that issimilar to the original box and holds twice as much.6. Suppose you dilated a cylinder with height 3 cm and radius 6 cm bya factor of ᎏ13ᎏ.a. What is the volume of the original cylinder?b. What is the volume of the scaled cylinder?c. Compare the volumes. Tell how the change in volume seems to berelated to the scale factor.7. This is a sketch of a net for a pyramid.a. Sketch and label a net for a similarpyramid, dilated by a scale factorof ᎏ12ᎏ.b. Find the surface area of theoriginal pyramid.c. Find the surface area ofthe dilated pyramid.d. Compare the surface areas byfilling in the blank: The newsurface area is ____ times theoriginal surface area.3 cm6 cm13 cm13 cm13 cm13 cm10 cm10 cm12 cm450-517_07elMSMgr7 6/7/04 5:23 PM Page 507