Answer
$S'(8) = 128\pi \frac{cm^{2}}{min}$
Work Step by Step
$S(r) = 4\pi r^{2}$
Use the chain rule:
$\frac{dS}{dt} = \frac{d(4\pi r^{2})}{dr}$
$\frac{dS}{dt} = 8\pi r \times \frac{dr}{dt}$
$ \frac{dr}{dt} = 2 \frac{cm}{min}$
$\frac{dS}{dt} = 8\pi (8) \times 2$
$\frac{dS}{dt} = 64\pi \times 2$
$\frac{dS}{dt} = 128\pi \frac{cm^{2}}{min}$
