9. Use the method of mathematical induction to prove that, for \(n \in \mathbb { Z } ^ { + }\),
- \(\left( \begin{array} { c c } 2 & 1
- 1 & 0 \end{array} \right) ^ { n } = \left( \begin{array} { c c } n + 1 & n
- n & 1 - n \end{array} \right)\) - \(\mathrm { f } ( n ) = 4 ^ { n } + 6 n - 1\) is divisible by 3 .