已知方程组 $\left(\begin{array}{ccc}1 & 2 & 1 \\ 2 & 3 & a+2 \\ 1 & a & -2\end{array}\right)\left(\begin{array}{l}x_1 \\ x_2 \\ x_3\end{array}\right)=\left(\begin{array}{l}1 \\ 3 \\ 0\end{array}\right)$ 无解，则 $a=$
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