Đáp án+Giải thích các bước giải:

 `a)128-3(x+4)=23`

`=>3(x+4)=128-23`

`=>3(x+4)=105`

`=>x+4=105:3=35`

`=>x=35-4=31`

Vậy $x=31$

`b)[(4x+28).3+55]:5=35`

`=>(4x+28).3+55=35.5=175`

`=>(4x+28).3=175-55=120`

`=>4x+28=120:3=40`

`=>4x=40-28=12`

`=>x=12:4=3`

`c)846:[41-(2x-5)]=47`

`=>41-(2x-5)=846:47=18`

`=>2x-5=41-18=23`

`=>2x=23+5=28`

`=>x=28:2=14`

`d)x(x+1)=111222`

`=>x(x+1)=333.334`

`=>x=333`

Vậy $x=333$

`a, 128 - 3(x + 4) = 23`

$\Rightarrow$ `3(x + 4) = 128 - 23`

$\Rightarrow$ `3(x + 4) = 105`

$\Rightarrow$ `x + 4 = 105 : 3`

$\Rightarrow$ `x + 4 = 35`

$\Rightarrow$ `x = 35 - 4`

$\Rightarrow$ `x = 31`

$\text{Vậy}$ `x = 31`

_____________________

`b, [(4x + 28) . 3 + 55] : 5 = 35`

$\Rightarrow$ `[(4x + 28) . 3 + 55] = 35 . 5`

$\Rightarrow$ `(4x + 28) . 3 + 55 = 175`

$\Rightarrow$ `(4x + 28) . 3 = 175 - 55`

$\Rightarrow$ `(4x + 28) . 3 = 120`

$\Rightarrow$ `(4x + 28) = 120 : 3`

$\Rightarrow$ `4x + 28 = 40`

$\Rightarrow$ `4x = 40 - 28`

$\Rightarrow$ `4x = 12`

$\Rightarrow$ `x = 12 : 4`

$\Rightarrow$ `x = 3`

$\text{Vậy}$ `x = 3`

_______________________

`c, 864 : [41 - (2x - 5)] = 47`

$\Rightarrow$ `[41 - (2x - 5)] = 846 : 47`

$\Rightarrow$ `41 - (2x - 5) = 18`

$\Rightarrow$ `(2x - 5) = 41 - 18`

$\Rightarrow$ `2x - 5 = 23`

$\Rightarrow$ `2x = 23 + 5`

$\Rightarrow$ `2x = 28` 

$\Rightarrow$ `x = 28 : 2`

$\Rightarrow$ `x = 14`

$\text{Vậy}$ `x = 14`

________________

`d, x . (x + 1) = 111222`

$\Rightarrow$ `x . (x + 1) = 333 . 334`

$\text{Mà}$ `x ; (x + 1)` $\text{là hai số cách đều}$

$\Rightarrow$ `x = 333`

$\text{Vậy}$ `x = 333`

$\text{Bài 3:}$

Gọi số tự nhiên cần tìm là $\overline{abc3}$ 

`=>` Nếu bỏ chữ số 3 thì bằng $\overline{abc}$ 

$\Rightarrow$ `\overline{abc3} - \overline{abc} = 1992`

$\Rightarrow$ `\overline{abc} xx 10 + 3 - \overline{abc} = 1992`

$\Rightarrow$ `\overline{abc} xx 10 - \overline{abc} = 1992 - 3`

$\Rightarrow$ `\overline{abc} xx 10 - \overline{abc} = 1989`

$\Rightarrow$ `\overline{9abc} = 1989`

$\Rightarrow$ `\overline{abc}  = 1989 : 9`

$\Rightarrow$ `\overline{abc}  = 221`

$\Rightarrow$ `\overline{abc3}  = 2213`

$\text{Vậy số cần tìm là 2213}$