sequences and series - How to prove {$a_n$ } is increasing where $a_1 = sqrt{2}$ and $a_{n+1} = sqrt{ 2+ a_n}$

I already found out that this sequence is bounded above and $a_n <2 \forall n \in \mathbb Z_+ $

I think I'm missing a point as I can't think of a way to prove that the sequence is increasing.