y=log2(x/x+1)
2^y=x/x+1
(2^y)(x+1)=x
(2^y)*x+2^y=x
x*(1-2^y)=2^y
x=2^y/(1-2^y)
将x和y互换
得到它的反函数
y=2^x/(1-2^x)