b) `5x^2+4x-9=0`
`⇔5x^2+9x-5x-9=0`
`⇔x(5x+9)-(5x+9)=0`
`⇔(5x+9)(x-1)=0`
`⇔`\(\left[ \begin{array}{l}5x+9=0\\x-1=0\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}5x=-9\\x=1\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}x=-\dfrac{9}{5}\\x=1\end{array} \right.\)
Vậy `S={-\frac{9}{5};1}`
c) `2x^2-3x-5=0`
`⇔2x^2+2x-5x-5=0`
`⇔2x(x+1)-5(x+1)=0`
`⇔(x+1)(2x-5)=0`
`⇔`\(\left[ \begin{array}{l}x+1=0\\2x-5=0\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}x=-1\\2x=5\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}x=-1\\x=\dfrac{5}{2}\end{array} \right.\)
Vậy `S={-1;\frac{5}{2}}`
