3.11. Find the volume $V$ of a truncated cone with height $h$ and base radii $R$ and $r$.
3.11. The given cone is obtained by cutting a cone with height $x$ and base radius $r$ from a cone with height $x+h$ and base radius $R$. Therefore, $V=\pi\left(R^{2}(x+h)-r^{2} x\right) / 3$. Since $x: r=(x+h): R$, then $x=r h /(R-r)$ and $x+h=R h /(R-$ $-r)$. Consequently, $V=\pi\left(r^{2}+r R+R^{2}\right) h / 3$.