8. For \(n \in \mathbb { Z } ^ { + }\)prove that
- \(2 ^ { 3 n + 2 } + 5 ^ { n + 1 }\) is divisible by 3 ,
- \(\left( \begin{array} { r r } - 2 & - 1
9 & 4 \end{array} \right) ^ { n } = \left( \begin{array} { c c } 1 - 3 n & - n
9 n & 3 n + 1 \end{array} \right)\).