Cho M = -3(x – 4)(x – 2) + x(3x – 18) – 25; N = (x – 3)(x + 7) – (2x – 1)(x + 2) + x(x – 1). Chọn khẳng định đúng.

M = -3(x – 4)(x – 2) + x(3x – 18) – 25

= -3(x2 – 2x – 4x + 8) + x.3x + x.(-18) – 25

= -3x2 + 6x + 12x – 24 + 3x2 – 18x – 25

= (-3x2 + 3x2) + (6x + 12x – 18x) – 24 – 25

= -49

N = (x – 3)(x + 7) – (2x – 1)(x + 2) + x(x – 1)

= x.x + x.7 – 3.x – 3.7 – (2x.x + 2x.2 – x – 1.2) + x.x + x.(-1)

= x2 + 7x – 3x – 21 – 2x2 – 4x + x + 2 + x2 – x

= (x2 – 2x2 + x2) + (7x – 3x – 4x + x – x) – 21 + 2

= -19

Vậy M = -49; N = -19 => M – N = -30

Đáp án cần chọn là: B