giải pt x^4-10x^2-x+20=0

$x^4-10x^2-x+20=0$

$⇔(x^4-x^3-5x^2)+(x^3-x^2-5x)-(4x^2-4x-20)=0$

$⇔(x^2-x-5)(x^2+x-4)=0$

$⇔\left[ \begin{array}{l}x=\frac{1±\sqrt[]{21}}{2}\\x=\frac{-1±\sqrt[]{17}}{2}\end{array} \right.$

Vậy $S=${$\frac{1±\sqrt[]{21}}{2};\frac{-1±\sqrt[]{17}}{2}$}.