Cho A = (3x + 7)(2x + 3) – (3x – 5)(2x + 11); B = x(2x + 1) – x2(x + 2

* Hướng dẫn giải

A = (3x + 7)(2x + 3) – (3x – 5)(2x + 11)

= 3x.2x + 3x.3 + 7.2x + 7.3 – (3x.2x + 3x.11 – 5.2x – 5.11)

$$\begin{gathered} = 6x^2 + 9x + 14x + 21 – (6x^2 + 33x – 10x – 55) \\ = 6x^2 + 23x + 21 – 6x^2 – 33x + 10x + 55 = 76 \end{gathered}$$

$$\begin{gathered} B = x(2x + 1) – x^2(x + 2) + x^3 – x + 3 \\ = x.2x + x – (x^2.x + 2x^2) + x^3 – x + 3 \\ = 2x^2 + x – x^3 – 2x^2 + x^3 – x + 3 = 3 \end{gathered}$$

Từ đó ta có A = 76; B = 3 mà 76 = 25.3 + 1 nên A = 25B + 1

Đáp án cần chọn là: C