a=2^x
y=(a-1)/(a+1)
y-1=(a-1-a-1)/(a+1)=-2/(a+1)
a+1=-2/(y-1)
a=2^x=(-1-y)/(y-1)
x=log2[(-1-y)/(y-1)]
反函数y=log2[(-1-x)/(x-1)]