`13) ( x - 1 )/9 = 8/3`

`=> 3 . ( x - 1 ) = 9 . 8`

`=> 3 . ( x - 1 ) = 72`

`=> x - 1 = 72 : 3`

`=> x - 1 = 24`

`=> x = 24 + 1`

`=> x = 25`

Vậy `, x = 25 .`

`14) ( - x )/4 = ( - 9 )/x`

`=> ( - x ) . x = 4 . ( - 9 )`

`=> ( - x ) . x = - 36`

`=> x^2 = 36`

`=>` \(\left[ \begin{array}{l}x^2 = 6^2\\x^2 = ( - 6 )^2\end{array} \right.\) 

`=>` \(\left[ \begin{array}{l}x = 6\\x = - 6\end{array} \right.\) 

Vậy `, x ∈ { 6 ; - 6 }`

`15) ( - 2 )/3 . x + 1/5 = 3/10`

`=> ( - 2 )/3 . x = 3/10 - 1/5`

`=> ( - 2 )/3 . x = 1/10`

`=> x = 1/10 : ( - 2 )/3`

`=> x = ( - 3 )/20`

Vậy `, x = ( - 3 )/20 .`

Đáp án + Giải thích các bước giải:

 `13, (x-1)/9 = 8/3`

`<=> 3(x-1)=72`

`<=> x-1=24`

`<=> x=25`

Vậy `x=25`

`14, (-x)/4  =(-9)/x`

`<=> -x^2 = -36`

`<=> x^2=36`

`<=> x^2=(+-6)^2`

`<=> x=+-6`

Vậy `x=+-6`

`15, -2/3 x + 1/5 = 3/10`

`<=> -2/3x = 1/10`

`<=> x=-3/20`

Vậy `x=-3/20`