6 9 Geometry Circles

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6 9 Geometry Circles

  1. 1. 6-9 Geometry: Circles and Circumference<br />Today you will learn to calculate the circumference of circles.<br />
  2. 2. New Vocabulary<br />Circle: set of all points in a plane that are the same distance from a given point called the center.<br />Diameter(d): the distance across a circle through it’s center.<br />Radius (r): the distance from the center to any point on a circle.<br />Circumference (C): the distance around a circle.<br />
  3. 3. Circles<br />Center<br />.<br />
  4. 4. Circles<br />Radius<br />
  5. 5. Circles<br />Diameter<br />
  6. 6. Circles<br />The diameter of a circle is 2 times the radius.<br />
  7. 7. Circumference of a Circle<br />The circumference of a circle is equal to it’s diameter (d) times π , or 2 times it’s radius (r) times π. <br />An approximation for π is 3.14.<br />d=2r<br />
  8. 8. Circles<br />Example: Ferris Wheels<br />Find the circumference of the Ferris wheel that measures 450 ft across.<br />
  9. 9. Circles<br />Always start by writing the formula that you are going to use.<br /> C=πd<br /> C≈3.14(450)<br /> C≈1,413<br />So, the distance around the Ferris wheel is about 1,413 feet. <br />Replace π with 3.14 and d with 450<br />
  10. 10. Find the Circumference<br />r=10 in<br />
  11. 11. Find the Circumference<br />r=10 in<br />C=2πr<br /> C≈2(3.14)(10)<br /> C≈62.8 in<br />
  12. 12. Find the Circumference<br />22<br />7<br />You can also use<br />for π.<br />Find the circumference of a circle with radius 21 cm.<br />
  13. 13. Circumference<br />C=2πr<br /> C≈2( )( )<br /> C≈ 132 cm<br />3<br />22<br />7<br />21<br />1<br />1<br />
  14. 14. Circles<br />Practice (Round to tenths if needed)<br /> 1.diameter = 5 in<br />2. radius = 12 m<br />3. radius = 1 <br />3<br />4<br />
  15. 15. Homework<br />Practice workbook Lesson 6-9<br />Be sure to show all of your work step by step. This means to write down the formula and show the values you substitute before you multiply. Don’t forget the units in your answer.<br />No work = No credit<br />

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