计算$\left|\begin{array}{cccccc}
2 a & a^2 & 0 & \cdots & 0 & 0 \\
1 & 2 a & a^2 & \cdots & 0 & 0 \\
0 & 1 & 2 a & \cdots & 0 & 0 \\
\cdots & \cdots & \cdots \cdots \cdots \cdots \cdots \cdots & \\
0 & 0 & 0 & \cdots & 2 a & a^2 \\
0 & 0 & 0 & \cdots & 1 & 2 a
\end{array}\right| .$ 其中$n \ge2$