Name: ________________ Date: _________ Aim: How do we find the area & circumference of a circle? Do Now: Define the following and label on the circle. Diameter___________________________________ Radius_____________________________________ Circumference_______________________________ 1. The diameter of a circle is 8 inches. Find the radius. 2. The radius of a circle is 15 cm. Find the diameter. Geo Complete the following chart using the information from the class circleactivity: Diameter Circumference Diameter _______ = Circumference 50 40 30 20 10 Circumference formulas : π = _________ Practice: Find the circumference of a circle when : 1. the radius is 4, in terms of π . 2. the diameter is 14, to the nearest tenth. 3. Find the radius of a circle, to the nearest tenth, when the circumference is 40 . 4. Find the diameter of a circle when the circumference is 20 π . Area of a Circle GeoLink Area Area of Rectangle Substitute r and C Plug in Circumference Formula Solve Area of a Circle Formula Practice: Find the area of a circle when... 4. the radius is 4, to the nearest tenth. 6. the diameter is 12, to the nearest hundredth. 8. If the circumference is 12π, find the radius. 5. the diameter is 14, in terms of π. 7. the radius is 12, in terms of π. 9. If the area is 64π, find the diameter. 11. If the circumference is 10π, find the area, in terms of π. 12. If the area is 9π, find the circumference, in terms of π. 13. The side of a square is 20 cm long. What is the circumference of the circle when … a. the circle is inscribed within the square? b. the square is inscribed within the circle? 14. A designer needs to create prefectly circular necklaces. The necklaces each need to have a radius of 8 inches. What is the largest number of necklaces that can be made from 2000 inches of wire? 15. The stained glass window below is a semi circle. To the nearest tenth, what is the distance around the window? 16. The cost of painting the circular traffic sign shown below is $2.75 per square foot. How much, to the nearest cent, will it cost to paint the sign if its diameter is 30 inches? Sum it Up • The area of a circle is πr . • The circumference of a circle is πd or 2πr. 2

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