`x^4 - 3x^2 - 28 = 0` `x^3 - 6x + 9 = 0`

`1)  x^4 - 3x^2 - 28 = 0`

Đặt `t = x^2` ta được `t^2 - 3t - 28 = 0` `⇒` $\left[\begin{matrix} t=-4\\ t=7\end{matrix}\right.$

`⇔` $\left[\begin{matrix} x^2 = -4\\ x^2 =7\end{matrix}\right.$

`⇔` $\left[\begin{matrix} x∉R\\ x=-\sqrt{7}\\x=\sqrt{7}\end{matrix}\right.$

  Vậy `S = {-\sqrt{7};\sqrt{7}}`

`2)  x^3 - 6x + 9 =0`

`⇔ x^3 - 9x + 3x + 9 =0`

`⇔ x(x^2-9)+3(x+3)=0`

`⇔ x(x-3)(x+3)+3(x+3) = 0`

`⇔ (x+3)[x(x-3)+3] = 0`

`⇔ (x+3)(x^2 - 3x + 3) =0`

`⇔` $\left[\begin{matrix} x+3=0\\ x^2-3x+3=0\end{matrix}\right.$

`⇔` $\left[\begin{matrix} x=-3\\ x∉R\end{matrix}\right.$

Vậy `S = {-3}`