#### Explain solution RD Sharma class 12 chapter 12 Derivative as a Rate Measurer exercise multiple choise question 16 maths

$B.\; r=\frac{1}{\pi }units$

Hint:

The area of circle of radius r is defined by

$A(r)=\pi r^{2}$

Given:

$\frac{d A}{d t}=\frac{2 d r}{d t}$

Solution:

We get

$2 \pi r \frac{d r}{d t}=\frac{2 d r}{d t}\\ \\ \pi r=1 \\ \\ r=\frac{1}{\pi} unite$