这题怎么这么纠结……
f(x)=(a*2^x-1)/(2^x+1)是R上的奇函数==>f(0)=0==>a=1
f(x)=(2^x-1)/(2^x+1) ==> f^-1(x)=log2[(1+x)/(1-x)] > log2(1+x)/k (1>x>-1) ==> (1+x)/(1-x)>(1+x)/k ==> 1-x1-x x>=1 ==> 无解