Hướng dẫn trả lời:
`D = (2x - 1)^2 - 2.(2x-3)^2 + 4.`
`D = [(2x)^2 - 2.2x.1 + 1^2] - 2.[(2x)^2 - 2.2x.3 + 3^2] + 4.`
`D = (4x^2 - 4x + 1) - 2.(4x^2 - 12x + 9) + 4.`
`D = 4x^2 - 4x + 1 - 8x^2 + 24x - 18 + 4.`
`D = (4x^2 - 8x^2) + (- 4x + 24x) + (1 - 18 + 4).`
`D = - 4x^2 + 20x - 13.`
Đáp án:
`D = - 4x^2 + 20x - 13.`