Given f(x) = x2 – 4x + 9
⇒ f'(x) = 2x – 4 ≥ 0 for all x in Df
⇒ f is strictly increasing function.
⇒ f is one-one function.
Also, Rf = [5, ∞) = Co-domain
⇒ f is onto function.
Therefore, f is a bijective function.
Hence, its inverse is exists.
Let y = x2 – 4x + 5
⇒ x2 – 4x + (5 – y) = 0