` a) `

` x + 2/5 = [ -1 ] / 7 `

` x = [ -1 ] / 7 - 2 / 5 `

` x = [ -5 ] / [ 35 ] - [ 14 ] / [ 35 ] `

` x = [ -5 - 14 ] / [ 35 ] `

` x = [ -19 ] / [ 35 ] `

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` b) `

` | x + 1/2 | = 7/6 `

$\left[\begin{matrix} x + \dfrac{1}{2} = \dfrac{7}{6} \\ x+\dfrac{1}{2}=\dfrac{-7}{6}\end{matrix}\right.$

$\left[\begin{matrix} x = \dfrac{7}{6} - \dfrac{1}{2}\\ x=\dfrac{-7}{6}-\dfrac{1}{2}\end{matrix}\right.$

$\left[\begin{matrix} x = \dfrac{7}{6} - \dfrac{3}{6}\\ x=\dfrac{-7}{6}-\dfrac{3}{6}\end{matrix}\right.$

$\left[\begin{matrix} x = \dfrac{4}{6} \\ x =\dfrac{-10}{6}\end{matrix}\right.$

$\left[\begin{matrix} x = \dfrac{2}{3} \\ x =\dfrac{-5}{3}\end{matrix}\right.$

 Vậy ` x = 2/3 ` hoặc ` x = [ -5 ] / 3 `

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` c) `

` [ 2430 ] / [ 3^x ] = 10 `

` 3^x = [ 2430 ] / [ 10 ] `

` 3^x = 243 `

` 3^x = 3^5 `

` x = 5 `

 Vậy ` x = 5 `

Đáp án $*$ Giải thích các bước giải:

 `a)x + 2/5=-1/7`

`=>x=-1/7 -2/5`

`=>x=-5/35 + (-14/35)`

`=>x=-19/35`

Vậy `x=-19/35`

 `b)|x+1/2|=7/6`

`=>x+1/2=7/6` hoặc `x+1/2=-7/6`

TH1:

`x+1/2=7/6`

`=>x=7/6 -1/2`

`=>x=7/6 -3/6`

`=>x=4/6=2/3`

TH2:

`x+1/2=-7/6`

`=>x=-7/6 -1/2`

`=>x=-7/6 + (-3/6)`

`=>x=-10/6=-5/3`

Vậy `x=2/3` hoặc `x=-5/3`

 `c)2430/(3^x)=10`

`=>3^x=2430/10`

`=>3^x=243`

`=>3^x=3^5`

`=>x=5`

 Vậy `x=5`