The radius of a circle is increasing at the rate of 0.02cms

^{-1}. Find the rate at which the area is increasing when the radius of the circle is 7cm.-
**A.**

0.75cm^{2}S^{-1} -
**B.**

0.53cm^{2}S^{-1} -
**C.**

0.35cm^{2}S^{-1} -
**D.**

0.88cm^{2}S^{-1}

##### Correct Answer: Option D

##### Explanation

A = (pi)r^{2}, (frac{delta A}{delta r}) = 2πr

So, using (frac{delta A}{delta t}) = (frac {delta A}{delta r}) x (frac {delta A}{delta t})

= 2(pi)r x 0.02

= 2(pi) x 7 x 0.02

= 2 x (frac{22}{7}) x 0.02

= 0.88cm^{2}s^{-1}

There is an explanation video available below.