∫x arctanxdx
=1/2∫ arctanxdx^2
=x^2arctanx/2-1/2∫x^2darctanx
=x^2arctanx/2-1/2∫x^2/(1+x^2)dx
=x^2arctanx/2-1/2∫(1-1/(1+x^2))dx
=x^2arctanx/2-x/2+arctanx/2+C.