Area of Circles and Parts of Circles

Finding the area of a circle The area of a circle is found using the formula A = r2

Practice Finding the area of a circle A = 4.7089 cm2 or 14.79 cm2

Practice Finding the area of a circle A = 6.734 cm2 or 21.16 cm2

Practice Finding the area of a circle Find the area of the shaded region A = 36 - 9 = 27 u2 or 84.82 u2 3 6

Practice Finding the area of a circle Find the area of the circle if the area of the square = 144 m2 The circle has a diameter of 12 m so A = 36 m2 or 113.1 m2

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 cm2 - 25 cm2 or 21.46 cm2

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 cm2 - 4(9) cm2 or 30.9 cm2

Finding the radius when area is known Since the formula for the area of a circle is A = πr2, the radius can be determined if the area is known. If A = 49π, then r2 = 49. If r2 = 49, then r = 7.

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

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 = 14π

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

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