Plz fast.... 
Explain me this differentiation ?? fAST..:

Q.  $$\begin{gathered} \therefore v=\frac{1}{3}{\mathrm{\pi r}}^2\mathrm{h} \\ \Rightarrow \mathrm{v}=\frac{1}{3}{\mathrm{\pi r}}^2\left[12+\sqrt{144-{\mathrm{r}}^2}\right] \\ \mathrm{On} \mathrm{differentiating} \mathrm{with} \mathrm{respect} \mathrm{to} \mathrm{r}, \mathrm{we} \mathrm{have}. \\ \frac{\mathrm{dv}}{\mathrm{dr}}=\frac{1}{3}\mathrm{\pi }\left(2\mathrm{r}\right) \left[12+\sqrt{144-{\mathrm{r}}^2}\right]+\frac{1}{3}\mathrm{\pi }\left({\mathrm{r}}^2\right)\left[\frac{-2\mathrm{r}}{2\sqrt{144-{\mathrm{r}}^2}}\right] \\ = \frac{2}{3}\mathrm{\pi }\left[12\mathrm{r}+\mathrm{r}\sqrt{144-{\mathrm{r}}^2}\right]-\frac{1}{3}\mathrm{\pi }\left[\frac{-{\mathrm{r}}^3}{\sqrt{144-{\mathrm{r}}^2}}\right] \end{gathered}$$