∫[x(cosx)^2]dx
=(1/2)∫xcos2xdx+(1/2)∫xdx
=x^2/4+(1/4)∫xdsin2x
=x^2/4+(xsin2x)/4-(1/4)∫sin2xdx
=x^2/4+(xsin2x)/4+cos2x/8+c
定积分=π^2