e^x=y
∫（e^x-1) / (e^x +1)dx
=∫（y-1) / (y +1)/ydy
=∫（2/(y+1)-1/y)dy
=2ln(y+1)-ln(y)
=2ln(e^x+1)-ln(e^x)
=2ln(e^x+1)-x