Cho M = -3(x – 4)(x – 2) + x(3x – 18) – 25; N = (x – 3)(x + 7) – (2x – 1)(x

* Hướng dẫn giải

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

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

= -3$x^2$ + 6x + 12x – 24 + 3$x^2$ – 18x – 25

= (-3$x^2$ + 3$x^2$) + (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)

= $x^2$ + 7x – 3x – 21 – 2$x^2$ – 4x + x + 2 + $x^2$ – x

= ($x^2$ – 2$x^2$ + $x^2$) + (7x – 3x – 4x + x – x) – 21 + 2

= -19

Vậy M = -49; N = -19 => M – N = -30

Đáp án cần chọn là: B