9. Prove by induction, that for \(n \in \mathbb { Z } ^ { + }\),
- \(\left( \begin{array} { l l } 3 & 0
6 & 1 \end{array} \right) ^ { n } = \left( \begin{array} { c c } 3 ^ { n } & 0
3 \left( 3 ^ { n } - 1 \right) & 1 \end{array} \right)\), - \(\mathrm { f } ( n ) = 7 ^ { 2 n - 1 } + 5\) is divisible by 12 .