Question 17

The rate of change of the area of a circle with respect to its radius *r* at *r* = 6 cm is

(A) 10π (B) 12π (C) 8π (D) 11π

Answer

The area of a circle (*A)* with radius (*r*) is given by,

A = πr^{2}

Therefore, the rate of change of the area with respect to its radius *r* is

\begin{align}\frac{dA}{dr} = \frac{d}{dr}(\pi r^2) = 2\pi r\end{align}

∴When *r* = 6 cm,

\begin{align}\frac{dA}{dr} = 2\pi \times 6 =12 \pi\; cm^2/s\end{align}

Hence, the required rate of change of the area of a circle is 12π cm^{2}/s.

The correct answer is B.

Deepthi N
2017-02-21 19:07:22

How the units of rate of change of area with respect for cm2 /sec

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