11 Выполните действия:

а) \( \frac{7}{9} - \frac{5}{12} = \frac{28}{36} - \frac{15}{36} = \frac{13}{36} \); б) \( \frac{5}{6} + \frac{7}{8} = \frac{20}{24} + \frac{21}{24} = \frac{41}{24} \); в) \( \frac{13}{21} - \frac{11}{22} = \frac{286}{462} - \frac{231}{462} = \frac{55}{462} \).