Tính: 4/7+8/7;8/9-5/7

* Hướng dẫn giải

$$\begin{gathered} a. \frac{4}{7}+\frac{8}{7}=\frac{12}{7} \\ b. \frac{8}{9}-\frac{5}{7}=\frac{56}{63}-\frac{45}{63}=\frac{11}{63} \\ c. \frac{8}{15}\times \frac{5}{4}=\frac{40}{60}=\frac{2}{3} \\ d. \frac{86}{17}÷\frac{43}{34}=\frac{86}{17}\times \frac{34}{43}=4 \end{gathered}$$