a) $36x^2-49=0$
$⇒36x^2=49 $
$⇒x^2 = \frac{49}{36} $
$⇒ x = ± \frac{7}{6} $
b) $x^3-16x=0$
$⇔x.(x^2-16)=0$
$⇔\left[ \begin{array}{l}x=0\\x^2-16=0\end{array} \right.$
Ta có : $x^2-16=0$
$⇒x^2=16$
$⇒x = ± 4 $
c) $(x-1)(x+2)-x-2=0$
$⇒(x+2)(x-2)=0$
$⇔\left[ \begin{array}{l}x+2=0\\x-2=0\end{array} \right.$
$⇔ \left[ \begin{array}{l}x=-2\\x=2\end{array} \right.$
d) $3x^3-27x=0$
$⇒x^3-9x=0$
$⇒x(x^2-9)=0$
$⇔ \left[ \begin{array}{l}x=0\\x^2-9=0\end{array} \right.$
Ta có : $x^2-9=0$
$⇒x^2=9$
$⇒x=±3$
