Ta xét 2TH
TH1 : $\left\{ \begin{array}{l}x^2-7>0\\x^2-51<0\end{array} \right.$
$⇔ \left\{ \begin{array}{l}x^2>7\\x^2<51\end{array} \right.$
$⇔7<x^2<51$
$⇔x^2 ∈ ${$9,16,25,36,49$}
$⇔ x ∈ ${$±3,±4,±5,±6,±7$}
TH2 : $ \left\{ \begin{array}{l}x^2-7<0\\x^2-51>0\end{array} \right.$
$⇔ \left\{ \begin{array}{l}x^2<7\\x^2>51\end{array} \right.$ ( Vô lí )
Vậy : $ x ∈ ${$±3,±4,±5,±6,±7$}
