In Section 6.3 the discussion of finding volumes by means of cylindrical shells was incomplete because we did not show that the same result is obtained if the disk method is also applicable. Use integration by parts to prove that if $f$ is differentiable and either $f^{\prime}(x)>0$ on $[a, b]$ or $f^{\prime}(x)<0$ on $[a, b],$ and if $V$ is the volume of the solid.