$( x - 1 )( 3x + 1 ) = 0$
⇔ \(\left[ \begin{array}{l}x-1=0\\3x+1=0\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=1\\x=\dfrac{-1}{3}\end{array} \right.\)
$S =$ { $1; \dfrac{-1}{3}$ }
$( x - 1)(x+2)(x-3)=0$
⇔ $x - 1 = 0 ; x + 2 = 0 và x - 3 = 0$
⇔ $x = 1; x = -2; x = 3$
S = { $1;-2;3$ }