分部积分法：
∫ln(x^2+1)dx
=xln(x^2+1)-∫2x^2/(x^2+1）dx
=xln(x^2+1)-2∫[1-1/(x^2+1)]dx
=xln(x^2+1)-2[x-arctanx]+C
=xln(x^2+1)-2x+2arctanx+C