∫(0->π/2) sinx .(cosx)^2 dx=-∫(0->π/2) (cosx)^2 dcosx=-(1/3)[ (cosx)^3]|(0->π/2)=1/3
原函数1/3cos^3x，代入答案-1/3
原式=-∫(0,π/2) cos^2xd(cosx)=-(1/3)*cos^3x|(0,π/2)=1/3
