tìm nghiệm các phương trình sau: a, 2x^2 - 3x -5 = 0 b, x^2 - 6x + 8 = 0 c, 9x^2 - 12x + 4 = 0 d, -3x^2 + 4x - 4 = 0

$a)$ $2x^2\:-\:3x\:-5\:=\:0$

`⇔2x(x+1)−5(x+1)=0`

`⇔(x+1)(2x−5)=0`

\(⇔\left[ \begin{array}{l}x+1=0\\2x-5=0\end{array} \right.\) \(⇔\left[ \begin{array}{l}x=-1\\x=\dfrac{5}{2}\end{array} \right.\) 

$\text{Vậy pt có nghiệm S = {-1 ; $\dfrac{5}{2}$}}$

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$b)$ $x^2-6x+8=0$

$⇔\left(x−4\right)\left(x−2\right)=0$

\(⇔\left[ \begin{array}{l}x-4=0\\\:x-2=0\end{array} \right.\) \(⇔\left[ \begin{array}{l}x=4\\x=2\end{array} \right.\) 

$\text{Vậy pt có nghiệm S = {4 ; 2}}$

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$c)$ $9x^2  - 12x + 4 = 0$

`⇔(3x) ^2 −2(3x)(2)+2 ^2 =0 `

`⇔(3x−2) ^2 =0 `

`⇔3x−2=0`

`⇔3x=2`

$⇔x=\dfrac{2}{3}$

$\text{Vậy pt có nghiệm S = {$\dfrac{2}{3}$}}$

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$d)$ $-3x^2\:+\:4x\:-\:4\:=\:0$

`⇔−3x^2+4x+−4=0`

$⇔x=\dfrac{-\left(4\right)\pm \sqrt{\left(4\right)^2-4\left(-3\right)\left(-4\right)}}{2\left(-3\right)}$

$⇔x=\dfrac{-\left(4\right)\pm \sqrt{32}}{-6}$

$\text{Vậy pt vô nghiệm}$