Bài `2:`
`7)`
`4x^2 - 12x + 9 - y^2`
` = (4x^2 - 12x + 9) - y^2`
` = [ (2x)^2 - 2 . 2x . 3 + 3^2] - y^2`
` = (2x-3)^2 - y^2`
` = (2x-3-y)(2x-3+y)`
`8)`
`16x^2 - 4y^2 + 4y - 1`
`= 16x^2 - (4y^2 - 4y + 1)`
`= (4x)^2 - [ (2y)^2 - 2 . 2y . 1 + 1^2]`
` = (4x)^2 - (2y - 1)^2`
` = [ 4x - (2y-1)] . [4x + (2y-1)]`
` = (4x - 2y + 1)(4x + 2y - 1)`
`9)`
`25 - x^2 - 12x - 36`
` = 25 - (x^2 + 12x + 36)`
` = 5^2 - (x^2 + 2 . x . 6 + 6^2)`
` = 5^2 - (x+6)^2`
` = [ 5 - (x+6)] [ 5 + (x+6)]`
` = (5 - x - 6)(5+x+6)`
` = (-1-x)(x+11)`
