Đáp án:
`a, x^2 - 9 = x^2 - 3^2 = (x - 3)(x + 3)`
`b, 4x^2 - 25 = (2x)^2 - 5^2`
`= (2x - 5)(2x + 5)`
`c, 1/25 x^2 - 64 y^2 = (1/5 x)^2 - (8y)^2`
`= (1/5 x - 8y)(1/5 x + 8y)`
`d, x^2 + 6x + 9`
`= x^2 + 2.x.3 + 3^2`
`= (x + 3)^2`
`e, 9x^2 + 6xy + y^2`
`= (3x)^2 + 2.3x.y + y^2`
`= (3x + y)^2`
`f, x^2 - 4xy + 4y^2`
`= x^2 - 2.x.2y + (2y)^2`
`= (x - 2y)^2`
`g, 4x^2 + 1 - 4x = 4x^2 - 4x + 1`
`= (2x)^2 - 2.2x.1 + 1^2`
`= (2x - 1)^2`
