(H, \langle .,. \rangle) Hilbert space, and f_1, f_2 \in H.
Prove: Af= \langle f,f_1 \rangle f_1 + \langle f,f_2 \rangle f_2 is bounded linear operator, and calculate his norm.


...Idea:  |Af| \leq ||f|| ||f_1|| ||f_1|| + ||f|| ||f_2|| ||f_2|| (??)