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

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1. 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 $x=\{5;-5\}$

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2. a) `2/3x(x^2-4)=0`

   `⇒2/3x(x-2)(x+2)=0`

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

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

   Vậy `x=0; ±2`

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

   `x(x-3)=0`

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

   Vậy `x=0; 3`

   c) `x^2-25=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`

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