TeX source:

\frac{\mathrm{dV}}{\mathrm{dt}}=\frac{\mathrm{d}\left(\pi \mathrm{r}^{2} \mathrm{~h}\right)}{\mathrm{dt}}=\pi \frac{\mathrm{d}\left(\mathrm{r}^{2} \mathrm{~h}\right)}{\mathrm{dt}}=\pi\left\{\mathrm{r}^{2} \frac{\mathrm{dh}}{\mathrm{dt}}+\mathrm{h} \frac{\mathrm{d} \mathbf{r}^{2}}{\mathrm{dt}}\right\}