f(-x)=log1/2(1+ax)/(-x-1)=-f(x)=-log1/2(1-ax)/(x-1)=log1/2(x-1)/(1-ax)
(1+ax)/(-x-1)=(x-1)/(1-ax)
1-x^2=1-a^2x^2
a^2=1
a=1或-1
若a=1
则f(x)=log1/2(1-x)/(x-1)=log1/2(-1)
无意义
所以a=-1