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\left([tex]\begin{array}{ll}2 & 1 \\ 1 & 2\end{array}\right) X-X\left(\begin{array}{rr}1 & -1 \\ 1 & 1\end{array}\right)=\left(\begin{array}{rr}1 & 1 \\ 1 & -1\end{array}\right)$;[/tex]