Let R be the area of the larger circle and r the area of the smaller circle. Then the area between them is . Now differentiate both sides with respect to t and use the fact that and . What is R at that time?
In this problem there are two varying concentrical circles with a constant area between at 9pi in^2. The area of the larger circle has a rate of change of 10pi in^2/sec. How fast is the circumference of the smaller circle changing when it has an area of 16pi in^2?
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Let R be the area of the larger circle and r the area of the smaller circle. Then the area between them is . Now differentiate both sides with respect to t and use the fact that and . What is R at that time?