$a$) $x^2 - 16 = 0$
$⇔ x^2 = 16$
$⇔ x = ±4$
Vậy $x = ±4$
$b$) $x^3 - 25x = 0$
$⇔ x(x^2 - 25) = 0$
$⇒$ \(\left[ \begin{array}{l}x=0\\x=-5\\x=5\end{array} \right.\)
Vậy $x$ $∈$ `{0;-5;5}`
$c$) $x^2 + 4x = -4$
$⇔ x^2 + 4x + 4 = 0$
$⇔ x^2 + 2x + 2x + 4 = 0$
$⇔ (x+2)^2 = 0$
$⇔ x = -2$
Vậy $x=-2$
$d$) $x^3 + 2x = 0$
$⇔ x(x^2 + 2) = 0$
$⇒ x = 0$
Vậy $x=0$