x=sint,t从0到pi/2，原积分为积分cos^3tcostdt＝积分(1+cos2t)^2/4=积分3/8+cos2t/2+cos4t/8=3pi/16
利用分部积分法∫ x sin2x dx =-1/2∫ x d(cos2x) =-1/2[xcos2x-∫cos2x dx ] =-1/2xcos2x+1/2∫cos2x dx =-1/2xcos2x+1/4∫cos2x d(2x
16/35
