Chapter 6.3, Problem 20E

Disk method Let R be the region bounded by the following curves. Use the disk method to find the volume of the solid generated when R is revolved about the x-axis.

20. y = cos x on [0, π/2], y = 0, x = 0 (Recall that ${\cos }^{2}x=\frac{1}{2}(1+\cos 2x)$.)