1. 6-9 Geometry: Circles and Circumference Today you will learn to calculate the circumference of circles.

2. New Vocabulary Circle: set of all points in a plane that are the same distance from a given point called the center. Diameter(d): the distance across a circle through it’s center. Radius (r): the distance from the center to any point on a circle. Circumference (C): the distance around a circle.

3. Circles Center .

4. Circles Radius

5. Circles Diameter

6. Circles The diameter of a circle is 2 times the radius.

7. Circumference of a Circle The circumference of a circle is equal to it’s diameter (d) times π , or 2 times it’s radius (r) times π. An approximation for π is 3.14. d=2r

8. Circles Example: Ferris Wheels Find the circumference of the Ferris wheel that measures 450 ft across.

9. Circles Always start by writing the formula that you are going to use. C=πd C≈3.14(450) C≈1,413 So, the distance around the Ferris wheel is about 1,413 feet. Replace π with 3.14 and d with 450

10. Find the Circumference r=10 in

11. Find the Circumference r=10 in C=2πr C≈2(3.14)(10) C≈62.8 in

12. Find the Circumference 22 7 You can also use for π. Find the circumference of a circle with radius 21 cm.

13. Circumference C=2πr C≈2( )( ) C≈ 132 cm 3 22 7 21 1 1

14. Circles Practice (Round to tenths if needed) 1.diameter = 5 in 2. radius = 12 m 3. radius = 1 3 4

15. Homework Practice workbook Lesson 6-9 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. No work = No credit