$\text{ a) $\dfrac{3}{4}$. x + $\dfrac{1}{2}$ = 0 }$

$\Rightarrow$ $\text{ $\dfrac{3}{4}$. x = 0 - $\dfrac{1}{2}$ }$

$\Rightarrow$ $\text{ $\dfrac{3}{4}$. x = $\dfrac{-1}{2}$ }$

$\Rightarrow$ $\text{ x = $\dfrac{-1}{2}$ : $\dfrac{3}{4}$ }$

$\Rightarrow$ $\text{ x = $\dfrac{-2}{3}$ }$

  $\text{ Vậy x = $\dfrac{-2}{3}$ }$

$\text{ b) | 2x | + | x - 5 | = 9   (1) }$

$\text{ Ta có bảng xét dấu : }$

   x   |________0_________5__________

  2x  |____-___0____+____|_____+_____            

x - 5 |____-___|_____-____0_____+____

$\text{ + Nếu x < 0, thì (1) trở thành : }$

$\text{ -2x + [ -( x - 5 ) ] = 9 }$

$\Leftrightarrow$ $\text{ -2x - x + 5 = 9 }$

$\Leftrightarrow$ $\text{ -3x = 9 - 5 }$

$\Leftrightarrow$ $\text{ -3x = 4 }$

$\Leftrightarrow$ $\text{ x = $\dfrac{-4}{3}$     ( t/mãn ) }$

$\text{ + Nếu 0 $\le$ x < 5, thì (1) trở thành : }$

  $\text{ 2x + [ -( x - 5 ) ] = 9 }$

$\Leftrightarrow$ $\text{ 2x - x + 5 = 9 }$

$\Leftrightarrow$ $\text{ x + 5 = 9 }$

$\Leftrightarrow$ $\text{ x = 9 - 5 }$

$\Leftrightarrow$ $\text{ x = 4    ( t/mãn ) }$

$\text{ + Nếu x > 5, thì (1) trở thành : }$

  $\text{ 2x + ( x + 5 ) = 9 }$

$\Leftrightarrow$ $\text{ 2x + x + 5 = 9 }$

$\Leftrightarrow$ $\text{ 3x = 9 - 5 }$

$\Leftrightarrow$ $\text{ 3x = 4 }$

$\Leftrightarrow$ $\text{ x = $\dfrac{4}{3}$    ( loại vì ko thuộc khoảng đang xét ) }$

        $\text{ Vậy x ∈ { $\dfrac{-4}{3}$ ; 4 } }$

a) `3/4. x + 1/2 = 0`

 `3/4. x = 0 - 1/2`

 `3/4. x = −1/2` 

`x = −1/2 : 3/4`

 `x = −2/3`

  Vậy `x = −2/3`

b) | 2x | + | x - 5 | = 9   

 Nếu `x < 0`, thì  

`-2x + [ -( x - 5 ) ] = 9`

` -2x - x + 5 = 9`

 `-3x = 9 - 5`

 `-3x = 4`

` x = −4/3`     ( t/mãn )

+ `Nếu 0≤ x < 5, thì `

`  2x + [ -( x - 5 ) ] = 9`

 `2x - x + 5 = 9`

` x + 5 = 9`

 `x = 9 - 5`

` x = 4`   ( t/mãn )

 Nếu `x > 5`, thì 

   `2x + ( x + 5 ) = 9`

 `2x + x + 5 = 9`

`3x = 9 - 5`

`3x = 4`

` x = 4/3`  ( loại)