Bài 5: Tìm x, biết: a) 2/3x (x ² – 4) = 0 b) x ² – 3x = 0 c) x ² – 25 = 0

a) $\dfrac{2}{3}x(x^2-4)=0$

\(↔\left[ \begin{array}{l}\dfrac{2}{3}x=0\\x^2-4=0\end{array} \right.\)

\(↔\left[ \begin{array}{l}x=0\\x^2=4\end{array} \right.\)

\(↔\left[ \begin{array}{l}x=0\\x=2;-2\end{array} \right.\)

Vậy $x=\{0;2;-2\}$

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

$↔x(x-3)=0$

\(↔\left[ \begin{array}{l}x=0\\x-3=0\end{array} \right.\)

\(↔\left[ \begin{array}{l}x=0\\x=3\end{array} \right.\)

Vậy $x=\{0;3\}$

c) $x^2-25=0$

$↔x^2-5^2=0$

$↔(x-5)(x+5)=0$

\(↔\left[ \begin{array}{l}x-5=0\\x+5=0\end{array} \right.\)

\(↔\left[ \begin{array}{l}x=5\\x=-5\end{array} \right.\)

Vậy $x=\{5;-5\}$