Answer:
[tex]s = 4\pi {r}^{2} \\ \frac{ds}{dr} = 8\pi r =8\pi(2) = 16\pi \\ from \: chain \: rule \\ \frac{ds}{dr} = \frac{ds}{dt} . \frac{dt}{dr} \\ but \: \frac{ds}{dt} = 1 \\ 16\pi = 1 \times \frac{dt}{dr} \\ \frac{dt}{dr} = 16\pi \\ \frac{dr}{dt} = ( \frac{dt}{dr} ) ^{ - 1} \\ \frac{dr}{dt} = \frac{1}{16\pi} [/tex]
