计算$D_n=\left|\begin{array}{cccccc}
x & -1 & 0 & \cdots & 0 & 0 \\
0 & x & -1 & \cdots & 0 & 0 \\
0 & 0 & x & \cdots & 0 & 0 \\
\vdots & \vdots & \vdots & & \vdots & \vdots \\
0 & 0 & 0 & \cdots & x & -1 \\
a_n & a_{n-1} & a_{n-2} & \cdots & a_2 & a_1+x
\end{array}\right| $