Tìm x, biết : a, 28+2x= 35-(-13) b, 21-(29+ 4x) = -12 c, |3x +9|-15 =27

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1. a, 28+2x= 35-(-13)

   ⇒28+2x=48

   ⇒2x=20

   ⇒x=10

   b, 21-(29+ 4x) = -12

   ⇒29+4x=33

   ⇒4x=4

   ⇒x=1

   c, |3x +9|-15 =27

   ⇒|3x+9|=42

   ⇒\(\left[ \begin{array}{l}3x+9=42\\3x+9=-42\end{array} \right.\) =>\(\left[ \begin{array}{l}3x=33\\3x=-51\end{array} \right.\) =>\(\left[ \begin{array}{l}x=11\\x=-17\end{array} \right.\) 

   Vậy x=11 hoặc x=-17

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2. $a) 28+2x=35-(-13)$

   $28+2x=48$

   $2x=48-28$

   $2x=20$

   $x=20:2$

   $→ x=10$

   $\textrm{Vậy x=10}$

   $b) 21-(29+4x)=-12$

   $29+4x=21-(-12)$

   $29+4x=33$

   $4x=33-29$

   $4x=4$

   $x=4:4$

   $→ x=1$

   $\textrm{Vậy x=1}$

   $c) |3x+9|-15=27$

   $|3x+9|=27+15$

   $|3x+9|=42$

   $TH1: 3x+9=42$

   $3x=42-9$

   $3x=33$

   $x=33:3$

   $→ x=11$

   $TH2: 3x+9=-42$

   $3x=(-42)-9$

   $3x=51$

   $x=51:3$

   $→ x=-17$

   $\textrm{Vậy x=11 hoặc x=-17}$

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