原式=xarctanx-∫xdarctanx
=xarctanx-∫xdx/(1+x²)
=xarctanx-1/2∫dx²/(1+x²)
=xarctanx-1/2∫d(1+x²)/(1+x²)
=xarctanx-ln(1+x²)/2+C