a) $$rac{7}{12} \div \frac{8}{9} = rac{7}{12} \cdot \frac{9}{8} = rac{7 \cdot 9}{12 \cdot 8} = rac{7 \cdot 3}{4 \cdot 8} = \frac{21}{32}$$
Ответ: $$rac{21}{32}$$
b) $$rac{8}{11} \div \frac{4}{5} = rac{8}{11} \cdot \frac{5}{4} = rac{8 \cdot 5}{11 \cdot 4} = rac{2 \cdot 5}{11 \cdot 1} = \frac{10}{11}$$
Ответ: $$rac{10}{11}$$