原式= -∫xd[1/(1+e^x)]
= -x/(1+e^x)+∫[1/(1+e^x)]dx
= -x/(1+e^x)+∫[(1+e^x-e^x)/(1+e^x)]dx
= -x/(1+e^x)+∫1dx-∫(1/(1+e^x))d(1+e^x)
=-x/(1+e^x)+x-ln(1+e^x)+C