For this case we have the following function: f (x) = e ^ x Rewriting we have: y = e ^ x We apply natural logarithm to both sides: Ln (y) = Ln (e ^ x) For logarithmic and exponential properties: x = Ln (y) Rewriting: f (x) ^ - 1 = Ln (x) Answer: the correct inverse function is: f (x) ^ - 1 = Ln (x)