Cho M = 4(x + 1)2 + (2x + 1)2 – 8(x – 1)(x + 1) – 12x và N = 2(x – 1

* Hướng dẫn giải

Ta có

$$\begin{gathered} M = 4{\left(x+1\right)}^2+{\left(2x+1\right)}^2-8\left(x-1\right)\left(x+1\right)-12x \\ = 4(x^2 + 2x + 1) + (4x^2 + 4x + 1) – 8(x^2 – 1) – 12x \\ = 4x^2 + 8x + 4 + 4x^2 + 4x + 1 – 8x^2 +8 – 12x \\ =\left(4x^2+4x^2-8x^2\right)+\left(8x+4x-12x\right)+4+1+8 \\ = 13 \end{gathered}$$

$$\begin{gathered} N = 2{(x – 1)}^2 – 4{(3 + x)}^2 + 2x(x + 14) \\ =2\left(x^2-2x+1\right)-4\left(9+6x+x^2\right)+2x^2+28x \\ =2x^2-4x+2-36-24x-4x^2+2x^2+28x \\ = (2x^2 +2x^2 – 4x^2) + (-4x – 24x + 28x) + 2 – 36 \\ = -34 \end{gathered}$$

Suy ra M = 13, N = -34 ó 2M – N = 60

Đáp án cần chọn là: B