1. a) 5/6 + (-7/12); 6) -3/10 - 1/15; B) -3/5 * 5/9; г) 8/21 : (-4/7).

1. a) $$\frac{5}{6} + \left(-\frac{7}{12}\right) = \frac{5 \cdot 2}{6 \cdot 2} - \frac{7}{12} = \frac{10}{12} - \frac{7}{12} = \frac{3}{12} = \frac{1}{4}$$.

б) $$-\frac{3}{10} - \frac{1}{15} = -\frac{3 \cdot 3}{10 \cdot 3} - \frac{1 \cdot 2}{15 \cdot 2} = -\frac{9}{30} - \frac{2}{30} = -\frac{11}{30}$$.

в) $$- \frac{3}{5} \cdot \frac{5}{9} = - \frac{3 \cdot 5}{5 \cdot 9} = -\frac{15}{45} = -\frac{1}{3}$$.

г) $$\frac{8}{21} : \left(-\frac{4}{7}\right) = \frac{8}{21} \cdot \left(-\frac{7}{4}\right) = - \frac{8 \cdot 7}{21 \cdot 4} = -\frac{56}{84} = -\frac{2}{3}$$.

Ответ: а) 1/4; б) -11/30; в) -1/3; г) -2/3.