First, remember how does the graph of the function f(x) = tan(x) look:
For the inverse of a function to exist, the function has to be an injective function.
A function is injective if it passes the horizontal line test.
Since the function f(x) = tan(x) is periodic and its period is equal to π, its domain must be restricted to an interval of length π in order to pass the horizontal line test.
If we keep the piece of the graph that passes through the origin, we must restrict the domain of the tangent function to the interval (-π/2,π/2) for the function to be injective, and thus for the inverse of the function to be defined.
Therefore, in both cases the answer is:
[tex](-\frac{\pi}{2},\frac{\pi}{2})[/tex]
