设 $A=\left(\begin{array}{ccc}1 & 1 & 1 \\ -1 & 1 & 1 \\ 1 & -1 & 1\end{array}\right), B=\left(\begin{array}{ccc}1 & 2 & 1 \\ 1 & 3 & -1 \\ 2 & 1 & 4\end{array}\right)$, 计䈯: (1). $A^2-B^2 ;(2)(A-B)(A+B) ;(3)(A+B)(A-B)$.
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