Khẳng định nào dưới đây đúng?
M = 8(x – 1)(x2 + x + 1) – (2x – 1)3 + 7 và N = (x + 2)(x2 – 2x + 4) – x(x – 1)2 – 8
M = 8(x – 1)(x2 + x + 1) – (2x – 1)3 + 7
= 8(x3 – 1) – (8x3 – 12x2 + 6x – 1) + 7
= 8x3 – 8 – 8x3 + 12x2 – 6x + 1 + 7
= 12x2 – 6x
N = (x + 2)(x2 – 2x + 4) – x(x – 1)2 – 8
= x3 + 8 – x(x2 – 2x + 1) – 8
= x3 – x3 + 2x2 – x
= 2x2 – x
⇒ 6N = 6(2x2 – x) = 12x2 – 6x
Vậy M = 6N