$3x² +5x -7 = 0$
$⇔ (\sqrt{3}x + \dfrac{5\sqrt{3}}{6})² - \dfrac{109}{12} = 0$
$⇔ (\sqrt{3}x + \dfrac{5\sqrt{3}}{6} - \sqrt{\dfrac{109}{12}}).(\sqrt{3}x + \dfrac{5\sqrt{3}}{6} + \sqrt{\dfrac{109}{12}}) = 0$
$⇔ \left[ \begin{array}{l}\sqrt{3}x + \dfrac{5\sqrt{3}}{6} - \sqrt{\dfrac{109}{12}}=0\\\sqrt{3}x + \dfrac{5\sqrt{3}}{6} + \sqrt{\dfrac{109}{12}}=0\end{array} \right. ⇔ \left[ \begin{array}{l}x=\dfrac{-5 + \sqrt{109}}{6}\\x=\dfrac{-5 - \sqrt{109}}{6}\end{array} \right.$
$\text {Vậy S = {$\dfrac{-5 - \sqrt{109}}{6}$; $\dfrac{-5 + \sqrt{109}}{6}$}}$
