5.3 The Fundamental Theorem of Calculus/10

${\begin{aligned}g(r)=\int _{0}^{r}{\sqrt {x^{2}+4}}\,dx{\frac {d}{dr}}(g(r))={\frac {d}{dr}}\left[\int _{0}^{r}{\sqrt {x^{2}+4}}\,dx\right]=1\cdot {\sqrt {(r)^{2}+4}}-0\cdot {\sqrt {(0)^{2}+4}}={\sqrt {r^{2}+4}}\end{aligned}}$