5. (10 pt) Suppose that we have two operators A and B such that (Ala) = (a Bla) for all a) in the Hilbert space under
consideration. Prove that A = B.
(Recall the following definition: A = B if and only if (8|Ala) = (B|Ala) in all kets (a), (β) in the Hilbert space.)