`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`
$\text{Vậy số cần tìm là 2213}$
