Xét ptrinh
$2x^2 - 6x + 1 = 0$
$<-> (x\sqrt{2})^2 - 2 . x\sqrt{2} . \dfrac{3}{\sqrt{2}} + \dfrac{9}{2} - \dfrac{7}{2} = 0$
$<-> \left( x \sqrt{2} - \dfrac{3}{\sqrt{2}} \right)^2 = \dfrac{7}{2}$
$<-> x \sqrt{2} - \dfrac{3}{\sqrt{2}} = \pm \dfrac{\sqrt{14}}{2}$
$<-> x \sqrt{2} = \dfrac{3\sqrt{2} \pm \sqrt{14}}{2}$
$<-> x = \dfrac{3 \pm \sqrt{7}}{2}$
Vậy $S = \left\{ \dfrac{3 - \sqrt{7}}{2}, \dfrac{3 + \sqrt{7}}{2} \right\}$.
