`#Sad`
`a)`
`(x^2-9)(x+1) = 0`
`⇔ (x-3)(x+3)(x+1) = 0`
`⇔` \(\left[ \begin{array}{l}x-3=0\\x+3=0\\x+1=0\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x=3\\x=-3\\x=-1\end{array} \right.\)
`\text{Vậy S=}` `{3; -3; -1}`
`b)`
`x^2+4x-5 = 0`
`⇔ x^2+5x-x-5 = 0`
`⇔ (x^2+5x)-(x+5) = 0`
`⇔ x(x+5)-(x+5) = 0`
`⇔ (x-1)(x+5) = 0`
`⇔` \(\left[ \begin{array}{l}x-1=0\\x+5=0\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x=1\\x=-5\end{array} \right.\)
`\text{Vậy S=}` `{1; -5}`
`c)`
`x^2+9x+20 = 0`
`⇔ x^2+4x+5x+20 = 0`
`⇔ (x^2+4x)+(5x+20) = 0`
`⇔ x(x+4)+5(x+4) = 0`
`⇔ (x+5)(x+4)`
`⇔` \(\left[ \begin{array}{l}x+5=0\\x+4=0\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x=-5\\x=-4\end{array} \right.\)
`\text{Vậy S=}` `{-5; -4}`
`d)`
`x^2-x-20 = 0`
`⇔ x^2-5x+4x-20 = 0`
`⇔ (x^2-5x)+(4x-20) = 0`
`⇔ x(x-5)+4(x-5) = 0`
`⇔ (x+4)(x-5) = 0`
`⇔` \(\left[ \begin{array}{l}x+4=0\\x-5=0\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x=-4\\x=5\end{array} \right.\)
`\text{Vậy S=}` `{-4; 5}`
`e)`
`2x^2+7x+6 = 0`
`⇔ 2x^2+4x+3x+6 = 0`
`⇔ (2x^2+4x)+(3x+6) = 0`
`⇔ 2x(x+2)+3(x+2) = 0`
`⇔ (2x+3)(x+2) = 0`
`⇔` \(\left[ \begin{array}{l}2x+3=0\\x+2=0\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x=-\dfrac{3}{2}\\x=-2\end{array} \right.\)
`\text{Vậy S=}` `{-3/2; -2}`
`f)`
`3x^2+x-4 = 0`
`⇔ 3x^2+4x-3x-4 = 0`
`⇔ (3x^2-3x)+(4x-4) = 0`
`⇔ 3x(x-1)+4(x-1) = 0`
`⇔ (3x+4)(x-1) = 0`
`⇔` \(\left[ \begin{array}{l}3x+4=0\\x-1=0\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x=-\dfrac{4}{3}\\x=1\end{array} \right.\)
`\text{Vậy S=}` `{-4/3; 1}`
