就是求y=f-1(x)反函数
2^y=(1+x)/(1-x)=-(x-1+2)/(x-1)
=-1-2/(x-1)
2/(x-1)=-2^y-1
x-1=-2/(2^y+1)
x=1-2/(2^y-1)=(2^y-1)/(2^y+1)
所以f(x)=(2^x-1)/(2^x+1)