86. Выполните действие:

Решение:

1. \( \frac{4}{7} + \frac{2}{3} = \frac{12}{21} + \frac{14}{21} = \frac{26}{21} \)
2. \( \frac{7}{18} - \frac{1}{12} = \frac{14}{36} - \frac{3}{36} = \frac{11}{36} \)
3. \( \frac{3}{9} + 2\frac{1}{7} = \frac{1}{3} + \frac{15}{7} = \frac{7}{21} + \frac{45}{21} = \frac{52}{21} \)
4. \( \frac{5}{14} - \frac{1}{21} = \frac{15}{42} - \frac{2}{42} = \frac{13}{42} \)

Ответ: а) \( \frac{26}{21} \); б) \( \frac{11}{36} \); в) \( \frac{52}{21} \); г) \( \frac{13}{42} \).