5. Given the sequence of positive integers excluding all terms of the sequence $12 n^{2}-n:\left(n \in \mathbf{N}_{+}\right)$, the remaining terms arranged in ascending order form the sequence $\left\{a_{n}\right\}$. Try to find the general term formula for $a_{n}$.
(Answer: $a_{n}=2 k^{2}-k+[n-2 k(k-1)]=n+k$, where $\left.k=\left[\frac{1+\sqrt{2 n-1}}{2}\right]\right)$