令g(y)= 根号(16-y²)+5 ,p(y)= -根号(16-y²)+5
所以要求的体积为(积分区间为-4到4)
π∫(g(y))²dy - π∫(p(y))²dy
=π∫20*根号(16-y²)dy
=20π*8π
=160π²