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A sphere's radius changes with time. At what radius do surface area and volume change at equal numerical rates?
A4 units
B8 units
C2 units
D1 unit
Answer & Solution
Correct answer: C. 2 units
dS/dt = 8πr(dr/dt) and dV/dt = 4πr^2(dr/dt) are equal when 8πr = 4πr^2, so r = 2.
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