V=pi ∫ f^2(x)dx
V=pi∫ 16cos^2(x)dx – pi ∫ 4cos^2(x) dx = 8pi ∫ 2cos^2(x) dx– 2pi ∫ 2cos^2(x)dx =
8pi ∫ (cos(2x)+1 dx – 2pi ∫ (cos(2x)+1) dx=
8pi (sin(2x)*(1/2) +x) -2pi (sin(2x)*(1/2)-x) =
4pi*(sin(2x)-pi*sin(2x) -6pi*x =
[4pi*sin(2*(-pi/2)-pi*sin(2*(-pi/2)-6*pi*(-pi/2)]- [4pi*sin(2*0-pi*sin(2*0-6*pi*0]=
4*pi*0-pi*0+pi^2-4pi*0+pi*0+6pi*0=pi^2