Tìm a / b biết a / b : 1 / 2 + 2 / 7 = 5 / 7

* Hướng dẫn giải

$$\begin{gathered} \frac{\mathrm{a}}{\mathrm{b}}:\frac{1}{2}+\frac{2}{7}=\frac{5}{7} \\ \frac{\mathrm{a}}{\mathrm{b}}:\frac{1}{2}=\frac{5}{7}-\frac{2}{7} \\ \frac{\mathrm{a}}{\mathrm{b}}:\frac{1}{2}=\frac{3}{7} \\ \frac{\mathrm{a}}{\mathrm{b}}=\frac{3}{7}\mathrm{x}\frac{1}{2} \\ \frac{\mathrm{a}}{\mathrm{b}}=\frac{3}{14} \end{gathered}$$