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