Answer
$\dfrac{dA}{dt} = 2\pi r \dfrac{dr}{dt}$
Work Step by Step
Area of a circle, A can be determined as:$\pi r^2$,
Thus, A = $\pi r^2$
On differentiation , we get:
$\dfrac{dA}{dt}$ = $\dfrac{d\pi r^2}{dt}$
or, $\dfrac{dA}{dt} = 2\pi r \dfrac{dr}{dt}$
Hence, $\dfrac{dA}{dt} = 2\pi r \dfrac{dr}{dt}$
