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Area of Circles and Parts of Circles

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Finding the area of a circle The area of a circle is found using the formula A = r 2

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Practice Finding the area of a circle A = r 2 A = (2.17) 2 A = 4.7089 cm 2 or 14.79 cm 2

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Practice Finding the area of a circle A = r 2 A = (5.19/2) 2 A = 6.734 cm 2 or 21.16 cm 2

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Practice Finding the area of a circle 6 3 Find the area of the shaded region A = 36 - 9 = 27 u 2 or 84.82 u 2

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Practice Finding the area of a circle Find the area of the circle if the area of the square = 144 m 2 The circle has a diameter of 12 m so A = 36 m 2 or 113.1 m 2

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Practice Finding the area of a circle Find the area of the shaded region if the side of the square = 10 cm The area is 100 cm 2 - 25 cm 2 or 21.46 cm 2

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Practice Finding the area of a circle Find the area of the shaded region if the side of the square is 12 cm. The area is 144 cm 2 - 4(9 ) cm 2 or 30.9 cm 2

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Finding the radius when area is known Since the formula for the area of a circle is A = πr 2, the radius can be determined if the area is known. If A = 49π, then r 2 = 49. If r 2 = 49, then r = 7.

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Finding the radius when area is known Find the radius for each circle 1. A = 144π 2. A = 36π 3. A = 81π 4. A = 25π 5.A = 100π r = 12 r = 6 r = 9 r = 5 r = 10

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Finding the circumference when area is known To find the circumference when the area is known, first find the radius and then use it to find the circumference. A = 49π so r = 7 and d = 14 C = 14 π

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Finding the circumference when area is known Find the circumference for each circle 1. A = 144π 2. A = 36π 3. A = 81π 4. A = 25π 5.A = 100π C = 24π C = 12π C = 18π C = 10π C = 20π

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