$14x^3 - 14x\sqrt{10 + 2\sqrt{5}} = 0$

`<=>` $x^3 - x\sqrt{10 + 2\sqrt{5}} = 0$

`<=>` $x\left(x^2 - \sqrt{10 + 2\sqrt{5}}\right) = 0$

$\text{TH1:}$ `x = 0`

$\text{TH2:}$

$x^2 - \sqrt{10 + 2\sqrt{5}} = 0$

`<=>` $x^2 = \sqrt{10 + 2\sqrt{5}}$

`<=>` $x = \pm \sqrt{\sqrt{10 + 2\sqrt{5}}} = \pm \sqrt[4]{10 + 2\sqrt{5}}$

Vậy $S = \left\{0; \, -\sqrt[4]{10 + 2\sqrt{5}}; \, \sqrt[4]{10 + 2\sqrt{5}}\right\}$

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