# Find the rate of change of the area of a circle with respect to its radius r when $(b)\; r = 4 cm$

This is (b) part of the multi-part question q1

• If $y=f(x)$,then $\large\frac{dy}{dx}$ measures the rate of change of $y$ w.r.t $x$.
• $\big(\large\frac{dy}{dx}\big)_{x=x_0}$ represents the rate of change of $y$ w.r.t $x$ at $x=x_0$
Area of the circle =$\pi r^2\;cm^2$
$A=\pi r^2$
Differentiating w.r.t $r$ we get,