`D = (2x+3)^2 - (x-6)(x-7)`
`= [ (2x)^2 + 2 . 2x . 3 + 3^2] - (x . x - x . 7 - 6 . x + 6 . 7)`
` = (4x^2 + 12x + 9) - (x^2 - 7x - 6x + 42)`
` = 4x^2 + 12x + 9 - x^2 + 7x + 6x - 42`
` = (4x^2 - x^2) + (12x + 7x + 6x) + (9-42)`
` = 3x^2 + 25x - 33`
Vậy `D = 3x^2 + 25x - 33`
`F = (3x-2y)^2 + (x+5y)(y-6x)`
` = [ (3x)^2 - 2 . 3x . 2y + (2y)^2] + (x . y - x . 6x + 5y . y - 5y . 6x)`
` = (9x^2 - 12xy + 4y^2) + (xy - 6x^2 + 5y^2 - 30xy)`
` = 9x^2 - 12xy + 4y^2 + xy - 6x^2 + 5y^2 - 30xy`
` = (9x^2 - 6x^2) + (4y^2 + 5y^2) + (-30xy - 12xy + xy)`
` = 3x^2 + 9y^2-41xy`
Vậy `F =3x^2 + 9y^2-41xy`
`G = (9x-10y)^2 - 2 (5x + 8y)^2`
` = [ (9x)^2 - 2 . 9x . 10y + (10y)^2] - 2 [ (5x)^2 + 2 . 5x . 8y + (8y)^2]`
`= (81x^2 - 180xy + 100y^2) - 2 (25x^2 + 80xy + 64y^2)`
`= 81x^2 - 180xy + 100y^2 - 50x^2 - 160xy - 128y^2`
` = (81x^2 - 50x^2) - (180xy + 160xy) + (100y^2 - 128y^2)`
` = 31x^2 - 340xy - 28y^2`
Vậy `G = 31x^2 - 340xy - 28y^2`
