3. Вычислите: а) \(\frac{4}{9}\cdot\left(-\frac{3}{8}\right)\); б) \(\frac{5}{6}-\frac{4}{9}\cdot\frac{15}{21}\).

а) \(\frac49\cdot\left(-\frac38\right)=-\frac{12}{72}=-\frac16\).

б) \(\frac56-\frac49\cdot\frac{15}{21}=\frac56-\frac49\cdot\frac57=\frac56-\frac{20}{63}=\frac{105-40}{126}=\frac{65}{126}\).

Ответ: а) \(-\frac16\); б) \(\frac{65}{126}\).