$\text { Đáp án: }$
` x³ + 5x² - 4x - 20 = 0 `
` => x²(x + 5) - 4(x + 5) = 0 `
` => (x² - 4)(x + 5) = 0 `
` => ` \(\left[ \begin{array}{l}x²-4=0\\x+5=0\end{array} \right.\)
` => ` \(\left[ \begin{array}{l}x=-2\\x=2\\x=-5\end{array} \right.\)
` \text { Vậy } S = {-5 ; -2; 2 } `