\\ $The Fibbonaci Sequence $ f_1, f_2,..., f_n $ is defined by $ f_1 = f_2 = 1 $ and $ f_n = f_{n-1} + f_{n-2}, n \geq 3. \\\\ $Let $ Q = \begin{bmatrix} 1 & 1 \\ 1 & 0 \end{bmatrix}.$ Prove that $ Q^n = \begin{bmatrix} f_{n+1} & f_n \\ f_n & f_{n-1} \end{bmatrix} $ and hence prove that $ f_{3n} = f_{n+1}^3 + f_n^3 - f_{n-1}^3