Задание 2.197

a) $$\frac{1}{5} + \frac{1}{7} = \frac{1 \cdot 7 + 1 \cdot 5}{5 \cdot 7} = \frac{7+5}{35} = \frac{12}{35}$$

б) $$\frac{1}{3} + \frac{2}{7} = \frac{1 \cdot 7 + 2 \cdot 3}{3 \cdot 7} = \frac{7+6}{21} = \frac{13}{21}$$

в) $$\frac{3}{5} + \frac{5}{6} = \frac{3 \cdot 6 + 5 \cdot 5}{5 \cdot 6} = \frac{18+25}{30} = \frac{43}{30}$$

г) $$\frac{8}{9} - \frac{2}{5} = \frac{8 \cdot 5 - 2 \cdot 9}{9 \cdot 5} = \frac{40 - 18}{45} = \frac{22}{45}$$