Lời giải

Đáp án đúng là: D

• $\left( {5x + 5} \right)x = 5\left( {x + 1} \right)x = 5x\left( {x + 1} \right) \Rightarrow \frac{{5x + 5}}{{5x}} = \frac{{x + 1}}{x}$

• $\left( {x - 2} \right)\left( {x + 2} \right) = {x^2} - 2x + 2x - 4 = {x^2} - 4 \Rightarrow \frac{{{x^2} - 4}}{{x + 2}} = x - 2$

• $\left( {x + 3} \right)\left( {x - 3} \right) = {x^2} + 3x - 3x - 9 = {x^2} - 9 \Rightarrow \frac{{x + 3}}{{{x^2} - 9}} = \frac{1}{{x - 3}}$

• $5\,.\,5x = 25x \ne 5x + 5 \Rightarrow \frac{{5x + 5}}{{5x}} \ne 5$