a) $4x^2 - 25 = 0$

$\Leftrightarrow 4x^2 = 25$

$\Leftrightarrow x^2 = \frac{25}{4}$

$\Leftrightarrow x = \pm \frac{5}{2}$

b) $2x^2 + 9x = 0$

$\Leftrightarrow x(2x + 9) = 0$

$\Leftrightarrow x = 0$ hoặc $x = \frac{-9}{2}$

c) $x^2 + x - 30 = 0$

$\Delta = 1^2 - 4\times 1 \times (-30) = 121 \Rightarrow \sqrt{\Delta } = 11$

$\Leftrightarrow x_1 = \frac{-1 + 11}{2} = 5;\; x_2 = \frac{-1 - 11}{2} = 6$

d) $2x^2 - 3x - 5 = 0$

$\Delta = (-3)^2 - 4\times 2 \times (-5) = 49 \Rightarrow \sqrt{\Delta } = 7$

$\Leftrightarrow x_1 = \frac{-(-3) + 7}{4} = \frac{5}{2};\; x_2 = \frac{-(-3) - 7}{4} = -1$