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3.4 Area and Circumference 1 Circle A circle is a plane figure that consists of all points that lie the same distance from a fixed point. The fixed point that defines a circle is called the center. Center A diameter is a segment that passes through the center of the circle and has endpoints on the circle. Diameter Diameter A radius is a segment with one endpoint on the center and the other endpoint on the circle. Radius Radius Next Slide

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3.4 Area and Circumference 2 Finding the diameter and radius of the circle If d = diameter, and r = radius: The diameter is twice the radius. So d = 2 r. Also the radius is half of the diameter. So r = d 2. Examples: If the diameter is 12 feet, what is the radius? Answer: The radius is d 2, so 12 2 = 6 ft. If the radius is 17 m, what is the diameter? Answer: The diameter is r 2, so 17 2 = 34 m. Next Slide

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3.4 Area and Circumference 3 The perimeter of a circle is called its circumference. Circumference Example 1. Find the circumference of a circle if the radius is 7 ft. (Use 3.14 for ) The formula is C = d. If the radius is 7 ft., then the diameter = 14 ft. C = 3.14 14 =43.96 ft Answer: 87.92 ft Your Turn Problem #1 Find the circumference of a circle if the diameter is 28 ft. (Use 3.14 for )

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3.4 Area and Circumference 4 Area Example 2. Find the Area of a circle if the radius is 7 ft. (Use 3.14 for ) The formula is A = r 2 or A = r r. A = 3.14 49 =153.86 ft 2 Your Turn Problem #2 Find the Area of a circle if the diameter is 18 ft. (Use 3.14 for ) Answer: 254.34 ft 2 The End

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