Question 6

# The radius of a circle is increasing at the rate of 0.7 cm/s. What is the rate of increase of its circumference?

The circumference of a circle (C) with radius (r) is given by

C = 2πr.

Therefore, the rate of change of circumference (C) with respect to time (t) is given by,

\begin{align} \frac{dC}{dt}=\frac{dC}{dr}.\frac{dr}{dt}\;\;\; [By\; Chain\; Rule]\end{align}

\begin{align} =\frac{d}{dr}(2\pi r).\frac{dr}{dt}\end{align}
\begin{align} =2\pi.\frac{dr}{dt}\end{align}

It is given that

\begin{align} \frac{dr}{dt}=0.7\; cm/s\end{align}

Hence, the rate of increase of the circumference 2π(0.7)=1.4π cm/s

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