calculus - To prove a sequence is Cauchy

I have a sequence:
$a_{n}=\sqrt{3+ \sqrt{3 + ... \sqrt { 3} } }$ , it repeats $n$-times.

and i have to prove that it is a Cauchy's sequence.
So i did this:
As one theorem says that every convergent sequence is also Cauchy, so i proved that it's bounded between $\sqrt{3}$ and $3$ (with this one i am not sure, please check if i am right with this one.)And also i proved tat this sequence is monotonic. (with induction i proved this: $a_{n} \leq a_{n+1}$
so if it's bounded and monotonic, therefore it is convergent and Cauchy.
I am just wondering if this already proved it or not? And also if the upper boundary - supremum if you wish - is chosen correctly.
I appreciate all the help i get.

Answer

${ a }_{ n+1 }=\sqrt { a_{ n }+3 }$ $\Rightarrow \quad { a^{ 2 } }_{ n+1 }=a_{ n }+3$ as $n\rightarrow \infty$ $\Rightarrow \quad { a^{ 2 } }_{ n+1 }=a_{ n }+3$ $\quad x^{ 2 }=x+3\quad \Rightarrow$ $x^{ 2 }-x-3=0$ $and\quad it\quad$ convergents to the $x=\frac { 1+\sqrt { 13 } }{ 2 }$