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A circle's radius grows at 2 cm/s. How fast is its area growing when r = 5 cm?
A10 pi sq cm/s
B25 pi sq cm/s
C20 pi sq cm/s
D40 pi sq cm/s
Answer & Solution
Correct answer: C. 20 pi sq cm/s
dA/dt = 2 pi r dr/dt = 2 pi times 5 times 2 = 20 pi.
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