Let $\{a_{n}\}$ be a sequence which satisfy

$a_{1}=5$ and $a_{n=}\sqrt[n]{a_{n-1}^{n-1}+2^{n-1}+2.3^{n-1}} \qquad \forall n\geq2$

[b](a)[/b] Find the general fomular for $a_{n}$

[b](b)[/b] Prove that $\{a_{n}\}$ is decreasing sequences