Chọn câu đúng? (3x - 4)/4x^2y^5 + (9x + 4)/4x^2y^5 = 3/xy^5

* Hướng dẫn giải

+) $\frac{\text{3x}-\text{4}}{{\text{4x}}^{\text{2}}{\text{y}}^{\text{5}}}+\frac{\text{9x}+\text{4}}{{\text{4x}}^{\text{2}}{\text{y}}^{\text{5}}}=\frac{\text{3x-4}+\text{9x}+\text{4}}{{\text{4x}}^{\text{2}}{\text{y}}^{\text{5}}}=\frac{\text{12x}}{{\text{4x}}^{\text{2}}{\text{y}}^{\text{5}}}=\frac{\text{3}}{{\text{xy}}^{\text{5}}}$ nên A sai.

+) $\frac{\text{2x}+\text{5}}{\text{3}}+\frac{\text{x}-\text{2}}{\text{3}}=\frac{\text{2x}+\text{5}+\text{x-2}}{\text{3}}=\frac{\text{3x}+\text{3}}{\text{3}}=\text{x}+\text{1}$ nên B sai.

+) $$\begin{gathered} \frac{\text{x}+\text{8}}{\text{x}-\text{1}}-\frac{\text{2x}-\text{1}}{\text{x}-\text{1}}-\frac{\text{6x}+\text{2}}{\text{x}-\text{1}}=\frac{\text{x}+\text{8}-\text{(2x}-\text{1)}-\text{(6x}+\text{2)}}{\text{x}-\text{1}} \\ =\frac{\text{x}+\text{8}-\text{2x}+\text{1}-\text{6x}-\text{2}}{\text{x}-\text{1}} \\ =\frac{-\text{7x}+\text{7}}{\text{x}-\text{1}}=\frac{-\text{7(x}-\text{1)}}{\text{x}-\text{1}}=-\text{7} \end{gathered}$$ nên C sai.