a. $x^2-4=0$
⇔ $(x-2)(x+2)=0$
⇔ \(\left[ \begin{array}{l}x-2=0\\x+2=0\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=2\\x=-2\end{array} \right.\)
Vậy $S=\{-2;2\}$
b. $x^2-2x=24$
⇔ $x^2-2x-24=0$
⇔ $x^2+4x-6x-24=0$
⇔ $x(x+4)-6(x+4)=0$
⇔ $(x+4)(x-6)=0$
⇔ \(\left[ \begin{array}{l}x+4=0\\x-6=0\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=-4\\x=6\end{array} \right.\)
Vậy $S=\{-4;6\}$
