用分步积分
∫(xcosx)/(sinx)^3 dx
=∫(x)/(sinx)^3 dsinx
=-1/2∫(x) d(1/sin^2x)
=-1/2s/sin^2x+1/2∫1/sin^2xdx
=-1/2s/sin^2x+1/2∫csc^2xdx
=-1/2s/sin^2x-1/2cotx+C