Đáp án:

`19) 6x^2 + 7x - 3`

` = 6x^2 + 9x - 2x - 3`

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

` = (3x-1)(2x+3)`

`20) x^2 - xy - 2y^2`

` = x^2 + xy - 2xy - 2y^2`

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

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

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

` = x^2 - xy + 2xy - 2y^2`

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

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

`22) x^2 - 3xy - 2y^2`

` = x^2 - 2 . x . 3/2 y + 9/4 y^2 - 17/4 y^2`

` = (x - 3/2 y)^2 - ( (sqrt{17} y)/2)^2`

` = (x - (3 - sqrt{17})/2 y )(x - (3 + sqrt{17})/2 y)`

`23) x^2 - xy - 6y^2`

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

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

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

`24) 2x^2 - 3xy - 2y^2`

` = 2x^2 - 4xy + xy - 2y^2`

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

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

`25) 6x^2 - xy - y^2`

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

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

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

`26) 2x^2 + 5xy + y^2`

` = 2(x^2 + 5/2 xy + (y^2)/2)`

` = 2[ x^2 + 2 . x . 5/4 y + 25/16 y^2 - 17/16 y^2]`

` = 2[ (x + 5/4 y)^2 - 17/16 y^2]`

` = 2(x + (5 - sqrt{17})/4 y)(x + (5 + sqrt{17})/4 y)`