令x=sint,原式=∫sintcost*tcostdt=∫t(sintcost^2)dt=t(-1/3cost^3)-∫-1/3cost^3dt
=-tcost^3/3+∫1/3(1-sint^2)costdt=-tcost^3/3+∫1/3(cost-sint^2cost)dt
=-tcost^3/3+1/3sint-1/9sint^3