Вычислите по действиям:(3/5-1/7)-(13/15-1/2)-1/21 (25/28-5/14)-(5/7-1/4)-1/28...

1. $( \frac{3}{5} - \frac{1}{7} )-( \frac{13}{15}- \frac{1}{2})- \frac{1}{21} = \frac{16}{35} - \frac{11}{30} - \frac{1}{21}= \frac{96-77-10}{210} =\frac{9}{210} = \frac{3}{70}$
2. $(\frac{25}{28}-\frac{5}{14})-(\frac{5}{7}-\frac{1}{4})- \frac{1}{28} =\frac{15}{28}-\frac{13}{28}-\frac{1}{28}=\frac{1}{28}$
3. $(\frac{35}{36}-\frac{5}{8})-(\frac{17}{18}-\frac{7}{9})- \frac{5}{72} =\frac{25}{72}-\frac{3}{18}-\frac{5}{72}=\frac{8}{72}= \frac{1}{9}$