Hướng dẫn trả lời:
1.
a) `-x^2 + 6x - 9 = - (x^2 - 6x + 9) = - (x^2 - 2.x.3 + 3^2) = - (x - 3)^2.`
b) `-9x^2-12x-4 = - (9x^2 + 12x + 4) = - [(3x)^2 + 2.3x.2 + 2^2] = - (3x + 2)^2.`
c) `(2x + 1)^2 - 2.(2x + 1) + 1 = (2x + 1)^2 - 2.(2x + 1).1 + 1^2 = [(2x + 1) - 1]^2`
`= (2x + 1 - 1)^2 = (2x)^2 = 4x^2.`
d) `(3x - 2y)^2 + 2.(3x - 2y) + 1 = (3x - 2y)^2 + 2.(3x - 2y).1 + 1^2 = [(3x - 2y) + 1]^2`
`= (3x - 2y + 1)^2.`