1. How do you find the circumference of a circle? For Example A circle has a radius of 8 centimeters. What is the circumference of the circle?

2. In this lesson you will learn to find the circumference of a circle by exploring the ratio of the diameter and circumference.

3. Let’s Review The Constant of Proportionality (Unit Rate) Number of BooksPrice 2$6 4$12 5$15 Constant of Proportionality = 3

4. Let’s Review Diameter is the distance across a circle through the center. Radius is the distance from the center of the circle to the edge of the circle. d = 2rr =

5. Core Lesson Springfield Middle School built a track in the shape of a circle. The diameter of the track is 100 meters. What is the circumference of the track? Circumference is the perimeter, or distance around, a circle.

6. Let’s Review Core Lesson C = 25.12 inches d = 8 inches C = 15.7 inches d = 5 inches Constant of Proportionality = 3.14 AB

7. Let’s Review Core Lesson Pi, represented by the symbol π, is a constant ratio that relates circumference and diameter. Pi is approximated as 3.14 25.12 =8 x 3.14 15.7 = 5 x 3.14 C = π d or C = 2 π r

8. Let’s Review A Common Mistake When using the radius to find the circumference, remember to multiply it by 2 to get the diameter. Wrong C = π r Correct C = 2 π r

9. Let’s Review Core Lesson What is the circumference of a circle that has a radius of 8 centimeters? Do not approximate pi. C = 2 π r C = (2) π (8 cm) C = 16 π cm

10. In this lesson you have learned to find the circumference of a circle by exploring the ratio of the diameter and circumference.

11. Let’s Review Guided Practice A circle has a radius of 6 inches. What is the circumference of the circle? Do not approximate pi.

12. Let’s Review Extension Activities

13. Let’s Review Extension Activities Alison stated “The circumference of a circle is approximately 6 times the radius of the circle.” Is Alison’s statement correct? Explain your reasoning.

14. Let’s Review Quick Quiz A circle has a diameter of 8 meters. What is the circumference of the circle? Use 3.14 for pi. A circle has a radius of 7 feet. Find the circumference of the circle. Do not approximate pi.