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Suppose a forest fire spreads in a circle with radius changing at a rate of 5 feet per minute. At what rate is the area of the burning region increasing when radius reaches 200 feet?

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A = πr²
dR/dt = 5 (given)
dA/dr = dA/dr * dr/dt = 2 pi r * 5

dA/dr is therefore 10πr when radius is 200 ft, dA/dr is 2000 pi ft/min

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