Let ({e}_{k } be an orthonormal sequence in a Hilbert space H, and let M=span({e}_{k }). show that for any x\in H we have a x \in \bar{M} if and only if x can be represented by \sum_{k = 1}^\infty {a}_{k} {e}_{k} with the coefficient {a}_{k} = <x, {e}_{k}.