g ( r ) = ∫ 0 r x 2 + 4 d x {\displaystyle g(r)=\int _{0}^{r}{\sqrt {x^{2}+4}}dx} d d x ∫ 0 r x 2 + 4 d x {\displaystyle {\frac {d}{dx}}\int _{0}^{r}{\sqrt {x^{2}+4}}dx} <math>\sqrt{x^2+4}
