There is a simple explanation for the remarkable result in Problem 35. Show that a horizontal plane

Question:

There is a simple explanation for the remarkable result in Problem 35. Show that a horizontal plane that intersects the region in Figure 12 and a sphere of radius b next to it will intersect in equal areas. Then apply Cavalieri's Principle for volume.
Consider the ring A of height 2h obtained from a sphere of radius a when a hole of radius c(c