Mathematics

Problem

If the area of a circle is

A=\pi r^{2}

, which of the following correctly gives

\dfrac{dA}{dt}

in terms of

r

and

\dfrac{dr}{dt}

? a)

\pi r\,\dfrac{dr}{dt}

2\pi r\,\dfrac{dr}{dt}

4\pi r\,\dfrac{dr}{dt}

3\pi r\,\dfrac{dr}{dt}