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C

- Hướng dẫn giải

Đáp án C

$$\begin{gathered} A=\frac{{\displaystyle \frac{2}{5}}+{\displaystyle \frac{2}{7}}+{\displaystyle \frac{2}{17}}-{\displaystyle \frac{2}{395}}}{{\displaystyle \frac{3}{5}}+{\displaystyle \frac{3}{7}}+{\displaystyle \frac{3}{17}}-{\displaystyle \frac{3}{395}}} \\ =\frac{2.{\displaystyle \frac{1}{5}}+2.{\displaystyle \frac{1}{7}}+2.{\displaystyle \frac{}{17}}-2{\displaystyle \frac{1}{395}}}{3.{\displaystyle \frac{1}{5}}+3.{\displaystyle \frac{1}{7}}+3.{\displaystyle \frac{1}{17}}-3.{\displaystyle \frac{1}{195}}} \\ =\frac{2\left({\displaystyle \frac{1}{5}}+{\displaystyle \frac{1}{7}}+{\displaystyle \frac{1}{17}}-{\displaystyle \frac{1}{395}}\right)}{3\left(\frac{1}{5}+\frac{1}{7}+\frac{1}{17}-\frac{1}{395}\right)}=\frac{2}{3} \end{gathered}$$