∫arctanx dx只能分部积分了
=xarctanx -∫x*darctanx
=xarctanx -∫x *1/(1+x^2)dx
=xarctanx -1/2∫ 1/(1+x^2) d(1+x^2)
=xarctanx - ln(1+x^2)/2+C