Đáp án:+Giải thích các bước giải:

`1, x^2 - 5x + 6`

`= x^2 - 2x - 3x + 6`

`= x(x - 2) - 3(x - 2)`

`= (x - 2)(x - 3)`

`x^2 - 5x + 6`

`= (x^2 - 4x + 4) - x + 2`

`= (x - 2)^2 - (x - 2)`

`= (x - 2)(x - 2 - 1)`

`= (x - 2)(x - 3)`

`2, x^2 + 5x + 6`

`= x^2 + 2x + 3x + 6`

`= x(x + 2) + 3(x + 2)`

`= (x + 2)(x + 3)`

`x^2 + 5x + 6`

`= x^2 + 4x + 4 + x + 2`

`= (x + 2)^2 + (x + 2)`

`= (x + 2)(x + 2 + 1)`

`= (x + 2)(x + 3)`

`3, x^2 - 7x + 12`

`= x^2 - 3x - 4x + 12`

`= x(x - 3) - 4(x - 3)`

`= (x - 3)(x - 4)`

`x^2 - 7x + 12`

`= (x^2 - 8x + 16) + x - 4`

`= (x^2 - 2.x.4 + 4^2) + (x - 4)`

`= (x - 4)^2 + (x - 4)`

`= (x - 4)(x - 4 + 1)`

`= (x - 4)(x - 3)`

`4, x^2 + 7x + 12`

`= x^2 + 3x + 4x + 12`

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

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

`x^2 + 7x + 12`

`= (x^2 + 6x + 9) + x + 3`

`= (x + 3)^2 + (x + 3)`

`= (x + 3)(x + 3 + 1)`

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