Bài 18/ Tìm x biết: a/ (x+5) . (x – 4) = 0 b/ (x – 1) . (x – 3) = 0 c/ (3 – x) . ( x – 3) = 0 d/ x(x + 1) = 0

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1. $a)(x+5)(x-4)=0$

   `=>`\(\left[ \begin{array}{l}x+5=0\\x-4=0\end{array} \right.\) `=>`\(\left[ \begin{array}{l}x=-5\\x=4\end{array} \right.\)

   $b)(x-1)(x-3)=0$

   `=>`\(\left[ \begin{array}{l}x-1=0\\x-3=0\end{array} \right.\) `=>`\(\left[ \begin{array}{l}x=1\\x=3\end{array} \right.\)

   $c)(3-x)(x-3)=0$

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

   $d)x(x+1)=0$

   `=>`\(\left[ \begin{array}{l}x=0\\x+1=0\end{array} \right.\) `=>`\(\left[ \begin{array}{l}x=0\\x=-1\end{array} \right.\)

    

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2. `a)(x+5)(x-4)=0`

   `⇔x+5=0` or `x-4=0`

   `1)x+5=0⇔x=-5`

   `2)x-4=0⇔x=4`

   Vậy `x∈{-5;4}`

   `b)(x-1)(x-3)=0`

   `⇔x-1=0` or `x-3=0`

   `1)x-1=0⇔x=1`

   `2)x-3=0⇔x=3`

   Vậy `x∈{1;3}`

   `c)(3-x)(x-3)`

   `1)3-x=0⇔x=3`

   `2)x-3=0⇔x=3`

   Vậy `x=3`

   `d)x(x+1)=0`

   `1)x=0`

   `2)x+1=0⇔x=-1`

   Vậy `x∈{-1;0}`

    

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