<human>:  Why is it true that f(x) = f for all x in the function ‘f(x) = f’?
<bot>: That’s a good question. It’s true because f is an inverse of f. In other words, if f(x) = f then x = f(x). So, for example, if f(x) = x^2, then f(x) = x^2 for all x.