Tìm \(x\), biết: \[{\left( {5 - x} \right)^2} = 9\].

\[{\left( {5 - x} \right)^2} = 9\]

\[5 - x = \pm 3\]

TH1: \[5 - x = 3\]

             \[\begin{array}{l}x = 5 - 3\\x = 2\end{array}\]

TH2: \[5 - x = - 3\]

             \[\begin{array}{l}x = 5 - ( - 3)\\x = 8\end{array}\]

Vậy \[x \in \left\{ {2\,;\,\,8} \right\}\].