The rate of change of the area of a circle W. r. t.its radius r at r= 4 cm i s:

11 π c m^{2} / s e c .

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10 π c m^{2} / s e c .

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8 π c m^{2} / s e c .

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12 π c m^{2} / s e c .

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Solution

The correct option is B $8 π c m^{2} / s e c .$
Given radius of circle $= 4$ cm
A $=$ Area $= π r^{2}$
Differentiating w.r.t. r we get
$\frac{d A}{d r} = 2 π r$
Now. Given $r = 4$ cm then
$\frac{d A}{d r} = 8 π$ $c m^{2} / s e c$ .

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