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Đáp án:

`7)` `x = 20`

`8)` `x = 20`

`9)` `x = 7`

`10)` `x = 8`

`11)` `x = 18`

`12)` `x = 2`

Giải thích các bước giải:

`7)` `125 - 5 . x = 25`

`⇒` `- 5 . x = 25 - 125`

`⇒` `- 5 . x = - 100`

`⇒` `x = ( - 100 ) : ( - 5 )`

`⇒` `x = 50`

`Vậy` `x = 20`

`8)` `2 + x : 5 = 6`

`⇒` `x : 5 = 6 - 2`

`⇒` `x : 5 = 4`

`⇒` `x = 4 . 5`

`⇒` `x = 20`

`Vậy` `x = 20`

`9)` `5x - 5^2 = 10`

`⇒` `5x - 25 = 10`

`⇒` `5x = 10 + 25`

`⇒` `5x = 35`

`⇒` `x = 35 : 5`

`⇒` `x = 7`

`Vậy` `x = 7`

`10)` `10x + 2^2 . 5 = 10^2`

`⇒` `10x + 4 . 5 = 100`

`⇒` `10x + 20 = 100`

`⇒` `10x = 100 - 20`

`⇒` `10x = 80`

`⇒` `x = 80 : 10`

`⇒` `x = 8`

`Vậy` `x = 8`

`11)` `125 - 5 ( 4 + x ) = 15`

`⇒` `125 - 5 . 4 - 5 . x = 15`

`⇒` `125 - 20 - 5 . x = 15`

`⇒` `105 - 5 . x = 15`

`⇒` `- 5 . x = 15 - 105`

`⇒` `- 5 . x = - 90`

`⇒` `x = ( - 90 ) : ( - 5 )`

`⇒` `x = 18`

`Vậy` `x = 18`

`12)` `5 . 2^2 + ( x + 3 ) = 5^2`

`⇒` `5 . 4 + x + 3 = 25`

`⇒` `20 + x + 3 = 25`

`⇒` `23 + x = 25`

`⇒` `x = 25 - 23`

`⇒` `x = 2`

`Vậy` `x = 2`