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设 $P=\left(\begin{array}{lll}1 & 0 & 0 \\ 1 & 1 & 0 \\ 1 & 1 & 1\end{array}\right), Q=\left(\begin{array}{lll}0 & 1 & 1 \\ 1 & 0 & 1 \\ 1 & 1 & 0\end{array}\right)$ 满足 $P X P+Q X Q=P X Q+Q X P +P^2-P Q$ ，求 $X$ ．
                        
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