计算行列式 $D_n=\left|\begin{array}{cccccccc}
9 & 5 & 0 & 0 & \cdots & 0 & 0 & 0 \\
4 & 9 & 5 & 0 & \cdots & 0 & 0 & 0 \\
0 & 4 & 9 & 5 & \cdots & 0 & 0 & 0 \\
\vdots & \vdots & \vdots & \vdots & & \vdots & \vdots & \vdots \\
0 & 0 & 0 & 0 & \cdots & 4 & 9 & 5 \\
0 & 0 & 0 & 0 & \cdots & 0 & 4 & 9
\end{array}\right|$