tính rồi rút gọn a) (2/5) + (3/6) ; b) (21/30) - (9/30) ; c) (5/8)*(4/7)

* Hướng dẫn giải

a) $\frac{2}{5}+\frac{3}{6}=\frac{12}{30}+\frac{15}{30}=\frac{27}{30}=\frac{9}{10}$

b) $\frac{21}{30}-\frac{9}{30}=\frac{12}{30}=\frac{2}{5}$

c) $\frac{5}{8}\times \frac{4}{7}=\frac{20}{56}=\frac{5}{14}$

d) $\frac{7}{9}÷\frac{11}{6}=\frac{7}{9}\times \frac{6}{11}=\frac{42}{99}=\frac{14}{33}$