y=(2^x-1)/(2^x+1)=(2^x+1-2)/(2^x+1)=1-2/(2^x+1)
1-y=2/(2^x+1) 2^x+1=2/(1-y)
2^x=2/(1-y) -1=(1+y)/(1-y)
x=log(下角2）（1+y)/(1-y)