e^x-ax>=1
e^x>=1+ax
取对数
x>=ln(1+ax)
令g(x)=x
h(x)=ln(1+ax)
g'(x)=1
h'(x)=a/(1+ax)
要使g'(x)>=h'(x)
1>=a/(1+ax)
a(1-x)0时即x1/(1-x)
x=1时
a