GROVE PRELIM 2U/3U BOOK - TEST YOURSELF 8 - Q13

Regarding $\pi$ as a constant and differentiating with respect to $x$.

$${dS\over dr}=2\times 4\pi r^{2-1}$$
$${dS\over dr}=8\pi r^{1}$$

$${dS\over dr}=8\pi r$$

C'est fini.

JH