I was just wondering if inverse functions only apply to one-to-one functions?(Or a function who's domain has been restricted to act as a one-to-one function). Thanks.
Yes, the function has to be one-to-one in order to have an inverse. For example, f(x) = x2 is not one-to-one, so doesn't have an inverse. However, if you restrict the domain to, say, x ≥ 0, then this restricted-domain function does have an inverse.
Alternatively, you could restrict the domain to x ≤ 0, and that function would have an inverse.