GRE exam question

If the radius of a circle is decreased by 50%, find the percentage decrease in its area. A) 55% B) 65% C) 75% D) 85%

Your Answer

Let r1 = 100
Then r2 = 50

A1 = π × 1002 = 10000π
A2 = π × 502 = 2500π

Decrease in area = 10000π - 2500π = 7500π

Percentage decrease in area = 7500π×10010000π=75%

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