Tính thuận tiện:

* Hướng dẫn giải

a. $\frac{2}{5}+\frac{3}{7}+\frac{3}{5}+\frac{4}{7}=\left(\frac{2}{5}+\frac{3}{5}\right)+\left(\frac{3}{7}+\frac{4}{7}\right)=\frac{5}{5}+\frac{7}{7}=1+1=2$

b. $\frac{2}{3}\times \frac{4}{9}+\frac{2}{3}\times \frac{5}{9}=\frac{2}{3}\times \left(\frac{4}{9}+\frac{5}{9}\right)=\frac{2}{3}\times \frac{9}{9}=\frac{2}{3}\times 1=\frac{2}{3}$