2 For all positive integers \(n\), the terms of the sequence \(\left\{ u _ { n } \right\}\) are given by the formula \(u _ { n } = 3 n ^ { 2 } + 3 n + 7 ( \bmod 10 )\).
- Show that \(u _ { n + 5 } = u _ { n }\) for all positive integers \(n\).
- Hence describe the behaviour of the sequence, justifying your answer.