Đáp án:

`1) x^2 - x - y^2 - y`

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

`= (x - y)(x + y) - (x + y)`

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

`2) x^2 - y^2 + x - y`

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

`= (x - y)(x + y) + (x - y)`

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

`3) 3x - 3y + x^2 - y^2`

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

`= 3(x - y) + (x - y)(x + y)`

`= (3 + x + y)(x - y)`

`4) 5x - 5y + x^2 - y^2`

`= (5x - 5y) + (x^2 - y^2)`

`= 5(x - y) + (x - y)(x + y)`

`= (5 + x + y)(x - y)`

`5) x^2 - 5x - y^2 - 5y`

`= (x^2 - y^2) - (5x + 5y)`

`= (x - y)(x + y) - 5(x + y)`

`= (x - y - 5)(x + y)`

`6) x^2 - y^2 + 2x - 2y`

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

`= (x - y)(x + y) + 2(x - y)`

`= (x - y)(x + y + 2)`

`7) x^2 - 4y^2 + x + 2y`

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

`= (x - 2y)(x + 2y) + (x + 2y)`

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

`8) x^2 - y^2 - 2x - 2y`

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

`= (x - y)(x + y) - 2(x + y)`

`= (x + y)(x - y - 2)`

`9) x^2 - 4y^2 + 2x + 4y`

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

`= (x - 2y)(x + 2y) + 2(x + 2y)`

`= (x + y)(x - 2y + 2)`