Đáp án:
a) $3x^2-18x=0$
$⇔3x.(x-6)=0$
$⇒$\(\left[ \begin{array}{l}x=0\\x=6\end{array} \right.\)
b) $-4x^2+20=0$
$⇔-4(x^2-5)=0$
$⇒$\(\left[ \begin{array}{l}-4=0 \\x=± √5\end{array} \right.\)
c) $4x^2-12x+9=0$
$⇒x=$$\frac{3}{2}$
d) $3x^2-2x-8=0$
$⇒$\(\left[ \begin{array}{l}x=2\\x=\frac{-4}{3} \end{array} \right.\)
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