x mũ 4 - 2x mũ 2 = 0

`***`Lời giải`***`

`x^4-2x^2=0`

`<=>x^2(x^2-2)=0`

`<=>x^2(x-\sqrt{2})(x+\sqrt{2})=0`

`<=>`\(\left[ \begin{array}{l}x^2=0\\x-\sqrt{2}=0\\x+\sqrt{2}=0\end{array} \right.\)

`<=>`\(\left[ \begin{array}{l}x=0\\x=\sqrt{2}\\x=-\sqrt{2}\end{array} \right.\)

Vậy `S={0;\sqrt{2};-\sqrt{2}}`