原式=∫(0→π)x^2d(sinx) =x^2sinx|(0→π)-∫(0→π)sinx*2xdx =0+2∫(0→π)xd(cosx) =2xcosx|(0→π)-2∫(0→π)cosxdx =-2π-2sinx|(0→π) =-2π
