x * a = a * a
\(\Leftrightarrow\) x * a - a * a = 0
\(\Leftrightarrow\) a * (x - a) = 0
\(\Leftrightarrow\left[{}\begin{matrix}a=0\\x-a=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}a=0\\x=a\end{matrix}\right.\)
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