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设 $A =\left(\begin{array}{lll}
a & & \\
& b & \\
& & a
\end{array}\right), P =\left(\begin{array}{lll}
2 & 1 & 0 \\
0 & 2 & 1 \\
0 & 0 & 2
\end{array}\right)$, 计算 $P ^2 A P ^{-1}=$
                        
不再提醒