Mathematics

Problem

The area

A

of a circle of radius

r

changes as the radius changes over time. Which expression gives

\dfrac{dA}{dt}

in terms of

r

and

\dfrac{dr}{dt}

? a)

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

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

\dfrac{1}{2}\pi r\dfrac{dr}{dt}

\pi r\dfrac{dr}{dt}