8) 2x + 7 = x – 1 9) x + 3 = – 9 – 11 10) 5 – x = 17 – (– 5) 11) – x – 14 + 35 = – 23 12) (– 5).x = (– 6). (– 10) 13) 9.x = (– 12). (– 60) 14) (7 – x).(13 + x) = 0 15) – 9.[x + (– 2)] = 0

Đáp án:

`8)` `2x + 7 = x - 1`

`<=>` `2x - x = -7 - 1`

`=>` `x = -8`

Vậy `:` `x = -8`.

`9)` `x + 3 = -9 - 11`

`<=>` `x + 3 = -20`

`<=>` `x = -20 - 3`

`=>` `x = -23`

Vậy `:` `x = -23`.

`10)` `5 - x = 17 - ( - 5 )`

`<=>` `5 - x = 17 + 5`

`<=>` `5 - x = 22`

`<=>` `x = 5 - 22`

`=>` `x = -17`

Vậy `:` `x = -17`.

`11)` `-x - 14 + 35 = -23`

`<=>` `-x - 14 = -23 - 35`

`<=>` `-x -14 = -58`

`<=>` `-x = -58 + 14`

`<=>` `-x = -44`

`=>` `x = 44`

Vậy `:` `x = 44`.

`12)` `( - 5 ) . x = ( -6 ) . ( - 10 )`

`<=>` `(-5).x = 60`

`<=>` `x = 60 : ( -5 )`

`=>` `x = -12`

Vậy `:` `x = -12`.

`13)` `9 . x = ( - 12 ) . ( - 60 )`

`<=>` `9 . x = 720`

`<=>` `x = 720 : 9`

`=>` `x = 80`

Vậy `:` `x = 80`.

`14)` `( 7 - x ). ( 13 + x ) = 0`

`<=>` \(\left[ \begin{array}{l}7-x = 0\\13+x=0\end{array} \right.\) 

`=>` \(\left[ \begin{array}{l}x = 7\\x=-13\end{array} \right.\) 

Vậy `:` `x ∈ { 7 ; -13 }`.

`15)` `-9 . [ x + ( - 2 ) ] = 0`

`<=>` `x - 2 = 0`

`<=>` `x = 0 + 2`

`=>` `x = 2`

Vậy `:` `x = 2`.