Question

If f^-1(x) is the inverse of f(x), which statement must be true?

Answer 1

The range of f⁻¹(x) is the domain of f(x) and vice versa
The domain of f⁻¹(x) is the range of f(x) and vice versa
and for every x of te domain: f(f⁻¹(x)) = x and f⁻¹(f(x)) = x

Answer 2

F(f-1x)   will be equal to x

take a simple example 

f(x) = 2x

so x = f(x) / 2

 f-1(x) = x/2

f(f-1x) =  2 ( x/2)  = 2 * x/2 = x