Решение:

1. a) $$ \frac{7}{12} \div \frac{8}{9} = \frac{7}{12} \times \frac{9}{8} = \frac{7 \times 9}{12 \times 8} = \frac{63}{96} = \frac{21}{32} $$
2. b) $$ \frac{8}{11} \div \frac{4}{5} = \frac{8}{11} \times \frac{5}{4} = \frac{8 \times 5}{11 \times 4} = \frac{40}{44} = \frac{10}{11} $$