Examples:
f := x*sin(x)^2
plot( f, x=0..Pi )
xvol := int( Pi*f^2, x=0..Pi )
xvol_num := evalf(xvol)
yvol := int( 2*Pi*x*f, x=0..Pi )
yvol_num := evalf(yvol)
f := x*sin(x)^2
plot( f, x=0..Pi )
xvol := int( Pi*f^2, x=0..Pi )
xvol_num := evalf(xvol)
yvol := int( 2*Pi*x*f, x=0..Pi )
yvol_num := evalf(yvol)