Tính :

* Hướng dẫn giải

$$\begin{gathered} \frac{2}{7}+\frac{3}{5}=\frac{10}{35}+\frac{21}{35}=\frac{31}{35} \\ \frac{31}{35}-\frac{2}{7}=\frac{31}{35}-\frac{2\times 5}{7\times 5}=\frac{31}{35}-\frac{10}{35}=\frac{21}{35} \\ \frac{31}{35}-\frac{3}{5}=\frac{31}{35}-\frac{3\times 7}{5\times 7}=\frac{31}{35}-\frac{21}{35}=\frac{10}{35} \\ \frac{3}{5}+\frac{2}{7}\equiv \frac{3\times 7}{5\times 7}+\frac{2\times 5}{7\times 5}=\frac{21}{35}+\frac{10}{35}=\frac{31}{35} \end{gathered}$$