∫xsin2xdx =1/2*∫xsin2xd2x =-1/2*∫xdcos2x =-1/2*xcos2x+1/2*∫cos2xdx =-1/2*xcos2x+1/4*∫cos2xd2x =-1/2*xcos2x+1/4*sin2x+C 把积分限代入 所以原式=-π/2
