Obviously, it is not easy to calculate the volume of one revolution of D2. We can use (D2+D3) to subtract the volume of D2 rotation from the volume of D2 rotation. So how do you get V3? Select a short section of dy, which will form a disk after rotating around the Y axis. The thickness of the disk is h=dy, and the radius of the disk is R = (y/2) (1/2), so the cross-sectional area of the disk is S = π r (2) = y π/2, and the volume of the disk is V=Sh. Infinite division will make dy→0, and countless others.