$\text{ 2$x^{4}$ - 10$x^{2}$ = 4 ( I )}$
$\text{ <=> 2$x^{4}$ - 10$x^{2}$ - 4 = 0}$
$\text{ Đặt $x^{2}$ = t ( t ≥ 0 ) }$
$\text{ ( I ) => 2$t^{2}$ - 10t - 4 = 0 }$
$\text{ Δ = $( -10 )^{2}$ - 4 . 2 . ( -4 ) = 132}$
$\text{ Δ > 0 => Phương trình có hai nghiệm phân biệt : }$
$\text{ $t_1$ = $\dfrac{- ( -10 ) + \sqrt{ 132 } }{2 . 2}$ = $\dfrac{5 + \sqrt{33}}{2}$ ( TM ) }$
$\text{ $t_2$ = $\dfrac{- ( -10 ) - \sqrt{ 132} }{2 . 2}$ = $\dfrac{5-\sqrt{33} }{2}$ ( Loại ) }$
$\text{ Với $t_1$ = $\dfrac{5+\sqrt{33} }{2}$ }$
$\text{ => $x_1$ = $\sqrt{(\dfrac{5+\sqrt{33}}{2}) }$ }$
$\text{ $x_2$ = - $\sqrt{(\dfrac{5+\sqrt{33}}{2})}$ }$
$\text{ Vậy S = { $\sqrt{(\dfrac{5+\sqrt{33}}{2}) }$ ; - $\sqrt{(\dfrac{5+\sqrt{33}}{2})}$ }}$
