=2∫xsin x/2 d(x/2)
=-2∫xd(cos x/2)
=-2[xcos x/2-∫cos x/2 dx]
=-2[xcos x/2-2∫cos x/2 d(x/2)]
=-2[xcos x/2-2sin x/2]
再把积分上下限代入即可