alleryThe rate of change of the area of a circle ($A = \pi r^2$) with respect to its radius, when $r = 5$, is:
A$5\pi$
B$10\pi$
C$2\pi$
D$25\pi$
Answer & Solution
Correct answer: B. $10\pi$
dA/dr = d/dr(πr²) = 2πr; at r=5 this is 10π.
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