a) $x^2 - 18 = 0$

$\Leftrightarrow x^2 = 18$

$\Leftrightarrow x = \pm \sqrt{18}$

b) $3x^2 - 15 = 0$

$\Leftrightarrow 3x^2 = 15$

$\Leftrightarrow x^2 = 5$

$\Leftrightarrow x = \pm \sqrt{5}$

c) $0,5x^2 + 3 = 0$

$\Leftrightarrow 0,5x^2 = -3$ (vô nghiệm)

d) $2x^2 + \sqrt{2} x = 0$

$\Leftrightarrow \sqrt{2}x(\sqrt{2} x + 1) = 0$

$\Leftrightarrow \left[ \begin{matrix}x = 0\\ x = \frac{1}{\sqrt{2}} = \frac{\sqrt{2}}{2}\end{matrix}\right.$

e) $-0,6x^2 + 2,4x = 0$

$\Leftrightarrow -0,6x(x - 4) = 0$

$\Leftrightarrow \left[ \begin{matrix}x = 0\\ x = 4\end{matrix}\right.$