Similarity day 1 sss, sas, aa

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Similarity day 1 sss, sas, aa

  1. 1. #3.19 Geo. Drill 3/5/14 • Given that the following two pentagons are similar, find x. 8 12 4 6 x 14 5 4 10
  2. 2. #3.19 Geo. Drill 3/5/14 • Given that the following two pentagons are similar, find x. 8 12 4 6 x 14 5 4 10
  3. 3. Geometry Drill •Can you list the 5 ways to prove triangles congruent
  4. 4. Objective • Students will use the similarity postulates to decide which triangles are similar and find unknowns.
  5. 5. Congruence vs. Similarity •Congruence implies that all angles and sides have equal measure whereas...
  6. 6. Congruence vs. Similarity •Similarity implies that only the angles are of equal measure and the sides are proportional
  7. 7. AA Similarity Postulate •Two triangles are similar if two pairs of corresponding angles are congruent
  8. 8. AA Similarity Postulate E B A C D F
  9. 9. SAS Similarity Postulate •Two triangles are similar if two pairs of corresponding sides are proportional and the included angles are congruent
  10. 10. SAS Similarity Postulate E B 12 8 A 4 C D 6 F
  11. 11. SSS Similarity Postulate •Two triangles are similar if all three pairs of corresponding sides are proportional.
  12. 12. SSS Similarity Postulate E B 9 13 A 7 C 21 27 D 39 F
  13. 13. Example 1 •∆APE ~ ∆DOG. If the perimeter of ∆APE is 12 and the perimeter of ∆DOG is 15 and OG=6, find the PE.
  14. 14. Example 2 Explain why ∆ABE ~ ∆ACD, and then find CD. Step 1 Prove triangles are similar. A  A by Reflexive Property of , and B  C since they are both right angles. Therefore ∆ABE ~ ∆ACD by AA ~.
  15. 15. Step 2 Find CD. Corr. sides are proportional. Seg. Add. Postulate. x(9) = 5(3 + 9) 9x = 60 60 20 x  9 3 Substitute x for CD, 5 for BE, 3 for CB, and 9 for BA. Cross Products Prop. Simplify. Divide both sides by 9.
  16. 16. Explain why ∆RSV ~ ∆RTU and then find RT. Step 1 Prove triangles are similar. It is given that S  T. R  R by Reflexive Property of . Therefore ∆RSV ~ ∆RTU by AA ~.
  17. 17. Check It Out! Example 3 Continued Step 2 Find RT. Corr. sides are proportional. Substitute RS for 10, 12 for TU, 8 for SV. RT(8) = 10(12) 8RT = 120 RT = 15 Cross Products Prop. Simplify. Divide both sides by 8.
  18. 18. Writing Proofs with Similar Triangles Given: 3UT = 5RT and 3VT = 5ST Prove: ∆UVT ~ ∆RST
  19. 19. Example 4 Continued Statements Reasons 1. 3UT = 5RT 1. Given 2. 2. Divide both sides by 3RT. 3. 3VT = 5ST 3. Given. 4. 4. Divide both sides by3ST. 5. RTS  VTU 5. Vert. s Thm. 6. ∆UVT ~ ∆RST 6. SAS ~ Steps 2, 4, 5
  20. 20. Given: M is the midpoint of JK. N is the midpoint of KL, and P is the midpoint of JL. Prove: ∆JKL ~ ∆NPM
  21. 21. Statements Reasons 1. M is the mdpt. of JK, N is the mdpt. of KL, and P is the mdpt. of JL. 1. Given 2. 2. ∆ Midsegs. Thm 3. 4. ∆JKL ~ ∆NPM 3. Div. Prop. of =. 4. SSS ~
  22. 22. Example 5: Engineering Application The photo shows a gable roof. AC || FG. ∆ABC ~ ∆FBG. Find BA to the nearest tenth of a foot. From p. 473, BF  4.6 ft. BA = BF + FA  6.3 + 17  23.3 ft Therefore, BA = 23.3 ft.
  23. 23. Check It Out! Example 5 What if…? If AB = 4x, AC = 5x, and BF = 4, find FG. Corr. sides are proportional. Substitute given quantities. 4x(FG) = 4(5x) FG = 5 Cross Prod. Prop. Simplify.
  24. 24. Conclusion •Similarity •AA •SAS •SSS
  25. 25. Classwork/Homework •Page 474-475 #’s 110, 17,18, 23, 24, 44-46

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