Question

How do you decide whether the relation `xy=9` defines a function?

Answer 1

Function means: at most one `y` for every `x`

Explanation:

We can rewrite as `y=9/x` or even `x=9/y`

In both cases we get only one answer, except when `x=0` or `y=0`
so we can say: `y=f(x)` and `x=g(y)`
graph{9/x [-22.8, 22.83, -11.4, 11.4]}