2. Выполните действия: a) $$\frac{7}{12} + \frac{5}{18}$$; б) $$\frac{7}{8} - \frac{5}{6}$$; в) $$\frac{4}{9} - \frac{2}{5} + \frac{7}{45}$$; $$\frac{5}{6} \times \frac{3}{20}$$; $$\frac{5}{14} : 28$$; $$5\frac{3}{5} \times \frac{3}{7}$$; $$\frac{7}{15} : \frac{14}{25}$$; $$2\frac{2}{9} : 1\frac{7}{9}$$

1. a) $$\frac{7}{12} + \frac{5}{18} = \frac{7 \times 3}{12 \times 3} + \frac{5 \times 2}{18 \times 2} = \frac{21}{36} + \frac{10}{36} = \frac{21+10}{36} = \frac{31}{36}$$
2. б) $$\frac{7}{8} - \frac{5}{6} = \frac{7 \times 3}{8 \times 3} - \frac{5 \times 4}{6 \times 4} = \frac{21}{24} - \frac{20}{24} = \frac{21-20}{24} = \frac{1}{24}$$
3. в) $$\frac{4}{9} - \frac{2}{5} + \frac{7}{45} = \frac{4 \times 5}{9 \times 5} - \frac{2 \times 9}{5 \times 9} + \frac{7}{45} = \frac{20}{45} - \frac{18}{45} + \frac{7}{45} = \frac{20-18+7}{45} = \frac{9}{45} = \frac{1}{5}$$
4. $$\frac{5}{6} \times \frac{3}{20} = \frac{5 \times 3}{6 \times 20} = \frac{15}{120} = \frac{1}{8}$$
5. $$\frac{5}{14} : 28 = \frac{5}{14} : \frac{28}{1} = \frac{5}{14} \times \frac{1}{28} = \frac{5 \times 1}{14 \times 28} = \frac{5}{392}$$
6. $$5\frac{3}{5} \times \frac{3}{7} = \frac{5\times 5 + 3}{5} \times \frac{3}{7} = \frac{28}{5} \times \frac{3}{7} = \frac{28 \times 3}{5 \times 7} = \frac{84}{35} = \frac{12 \times 7}{5 \times 7} = \frac{12}{5} = 2\frac{2}{5}$$
7. $$\frac{7}{15} : \frac{14}{25} = \frac{7}{15} \times \frac{25}{14} = \frac{7 \times 25}{15 \times 14} = \frac{175}{210} = \frac{5 \times 35}{6 \times 35} = \frac{5}{6}$$
8. $$2\frac{2}{9} : 1\frac{7}{9} = \frac{2\times 9 + 2}{9} : \frac{1\times 9 + 7}{9} = \frac{20}{9} : \frac{16}{9} = \frac{20}{9} \times \frac{9}{16} = \frac{20}{16} = \frac{5 \times 4}{4 \times 4} = \frac{5}{4} = 1\frac{1}{4}$$

Ответ: a) $$\frac{31}{36}$$; б) $$\frac{1}{24}$$; в) $$\frac{1}{5}$$; $$\frac{1}{8}$$; $$\frac{5}{392}$$; $$2\frac{2}{5}$$; $$\frac{5}{6}$$; $$1\frac{1}{4}$$