f(x)=(a*2^x-1)/(2^x+1)满足f(-x)=-f(x) ;f(0)=(a-1)/2=0
a=1
f(x)=(2^x-1)/(2^x+1);x∈R
f-1(x)=log2 (1+x)/(1-x);x∈(-1,1)
f-1(x)>log2(1+x) ;log2 (1+x)/(1-x)>log2(1+x)
log2(1-x)