计算
$$
D_n=\left|\begin{array}{cccccc}
x+y & x y & 0 & \cdots & 0 & 0 \\
1 & x+y & x y & \cdots & 0 & 0 \\
0 & 1 & x+y & \cdots & 0 & 0 \\
\cdots & \cdots & \cdots & \cdots & \cdots & \cdots \\
0 & 0 & 0 & \cdots & x+y & x y \\
0 & 0 & 0 & \cdots & 1 & x+y
\end{array}\right|
$$