∫x* cos²x dx
=1/2∫x* (1+cos2x)dx
=x^2/4+1/2∫xcos2xdx
=x^2/4+1/4∫xdsin2x
=x^2/4+1/4xsin2x-1/4∫sin2xdx
=x^2/4+1/4xsin2x+1/8cos2x+C