分部积分法
∫ ln(1+x²) dx
=xln(1+x²)-∫ xd[ln(1+x²)]
=xln(1+x²)-∫ [x*2x/(1+x²)]dx
=xln(1+x²)-2∫ x²/(1+x²)dx
=xln(1+x²)-2∫ [1-1/(1+x²)]dx
=xln(1+x²)-2x+2arctanx+C
C为任意常数