Chọn câu sai. (5x + 5)/5x = (x + 1)/x

* Hướng dẫn giải

Ta có

$\frac{\text{5x}+\text{5}}{\text{5x}}=\frac{\text{5(x}+\text{1)}}{\text{5x}}=\frac{\text{5(x}+\text{1):5}}{\text{5x:5}}=\frac{\text{x}+\text{1}}{\text{x}}$ nên A đúng, D sai.

$\frac{{\text{x}}^{\text{2}}-\text{9}}{\text{x}+\text{3}}=\frac{\text{(x}-\text{3)(x}+\text{3)}}{\text{(x}+\text{3)}}=\frac{\text{(x}-\text{3)(x}+\text{3):(x}+\text{3)}}{\text{(x}+\text{3):(x}+\text{3)}}=\text{x}-\text{3}$ nên B đúng

$\frac{\text{x}+\text{3}}{{\text{x}}^{\text{2}}-\text{9}}=\frac{\text{x}+\text{3}}{\text{(x}+\text{3)(x}-\text{3)}}=\frac{\text{(x}+\text{3):(x}+\text{3)}}{\text{(x}-\text{3)(x}+\text{3):(x}+\text{3)}}=\frac{\text{1}}{\text{x}-\text{3}}$ nên C đúng.

Đáp án cần chọn là: D