Find a formula for the inverse of the function f(x)=(1+e^x)/(1-e^x)

Let y = f(x)

y = (1 + e^x)/(1 - e^x)

Exchange places with x and y.

x = (1 + e^y)/(1 - e^y)

Now solve for y.

y = log[(x + 1)/(x - 1)]

Replace y with the inverse notation.

f^(-1) x = log[(x + 1)/(x - 1)]

Done!