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设 $\boldsymbol{A}^{-1}=\left(\begin{array}{cc}1 & 2 \\ 3 & -1\end{array}\right), \boldsymbol{B}^{-1}=\left(\begin{array}{cc}2 & -1 \\ 1 & 2\end{array}\right)$ ，则 $(\boldsymbol{A} \boldsymbol{B})^{-1}=$
                        
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