Выполните действия. a) $$\frac{3}{7} + \frac{2}{13}$$; б) $$\frac{8}{9} - \frac{7}{12}$$; в) $$\frac{11}{50} - \frac{3}{25} + \frac{1}{20}$$

a) $$\frac{3}{7} + \frac{2}{13} = \frac{3 \cdot 13 + 2 \cdot 7}{7 \cdot 13} = \frac{39 + 14}{91} = \frac{53}{91}$$

б) $$\frac{8}{9} - \frac{7}{12} = \frac{8 \cdot 4 - 7 \cdot 3}{36} = \frac{32 - 21}{36} = \frac{11}{36}$$

в) $$\frac{11}{50} - \frac{3}{25} + \frac{1}{20} = \frac{11 \cdot 2 - 3 \cdot 4 + 1 \cdot 5}{100} = \frac{22 - 12 + 5}{100} = \frac{15}{100} = \frac{3}{20}$$