Đặt tính rồi tính.

* Hướng dẫn giải

a) $\frac{5}{6}+\frac{7}{8}= \frac{40}{48}+\frac{42}{48}=\frac{82}{48}=\frac{41}{24}$

b)$\frac{5}{8}-\frac{2}{5}=\frac{25}{40}-\frac{16}{40}=\frac{25-16}{40}=\frac{9}{40}$

c) $\frac{9}{10}\times \frac{5}{6}=\frac{9\times 5}{10\times 6}=\frac{3\times 1}{2\times 2}=\frac{3}{4}$

d) $\frac{6}{5}:\frac{3}{7}= \frac{6}{5}\times \frac{7}{3}=\frac{6\times 7}{5\times 3}=\frac{2\times 7}{5\times 1}=\frac{14}{5}$