Решение:

1. а) $$rac{1}{5} + \frac{1}{7} = \frac{7}{35} + \frac{5}{35} = \frac{7+5}{35} = \frac{12}{35}$$
   Ответ: $$rac{12}{35}$$
2. б) $$rac{1}{3} + \frac{2}{7} = \frac{7}{21} + \frac{6}{21} = \frac{7+6}{21} = \frac{13}{21}$$
   Ответ: $$rac{13}{21}$$
3. в) $$rac{3}{5} + \frac{5}{6} = \frac{18}{30} + \frac{25}{30} = \frac{18+25}{30} = \frac{43}{30} = 1\frac{13}{30}$$
   Ответ: $$1\frac{13}{30}$$
4. г) $$rac{8}{9} - \frac{2}{5} = \frac{40}{45} - \frac{18}{45} = \frac{40-18}{45} = \frac{22}{45}$$
   Ответ: $$\frac{22}{45}$$