设X1>X2
F(X1)-F(X2)=In[(1+e^x1)/(1+e^x2)]+x1-x2
x1>x2
x1-x2>0
[(1+e^x1)/(1+e^x2)>1
In[(1+e^x1)/(1+e^x2)]>0
f(x1)-f(x2)>0
f(x)在R上单调递增