Calculus concavity

Question

Let f be the function defined by

f(x) = x + 9/x

on (-infinity, 0)

Find the derivative of f.

Answer 1

f(x) = x + 9/x

For me, it is easier to do the math when written this way:

f(x) = x + 9 * x^-1

f'(x) = 1 - 9 * x^-2 = 1 - 9/x²

This is defined everywhere except x = 0, so I am not clear on the significance of on "on (-infinity, 0)".

If x goes to infinity ( + or - ), f'(x) = 1