Решение:
а) $(-\frac{2}{3}) * (-\frac{1}{2}) * \frac{3}{4} = \frac{\overset{1}{\cancel{2}} * 1 * \overset{1}{\cancel{3}}}{\underset{1}{\cancel{3}} * \underset{1}{\cancel{2}} * 4} = \frac{1}{4}$;
б) $(-\frac{4}{5}) * (-\frac{3}{4}) * (-\frac{1}{2}) = -\frac{\overset{1}{\cancel{4}} * 3 * 1}{5 * \underset{1}{\cancel{4}} * 2} = -\frac{3 * 1}{5 * 2} = -\frac{3}{10}$.
Ответ:
а) $\frac{1}{4}$;
б) $-\frac{3}{10}$.