Ответ:
а) \(\frac{3}{7} \cdot \left(-\frac{14}{15}\right) = -\frac{42}{105} = -\frac{2}{5}\).
б) \(\frac{2}{3} : \frac{8}{9} = \frac{2}{3} \cdot \frac{9}{8} = \frac{3}{4}\).
\(\frac{5}{6} - \frac{3}{4} = \frac{10}{12} - \frac{9}{12} = \frac{1}{12}\).
Ответ: а) \(-\frac{2}{5}\); б) \(\frac{1}{12}\).
