Chapter 6.7, Problem 40ES

Use the Theorem of Pappus and the result of Exercise $39$ to find the volume of the solid generated when the region bounded by the $x\text{-axis}$ and the semicircle $y=\sqrt{a^2-x^2}$ is revolved about

(a) the line $y=-a$

(b) the line $y=x-a$ .