Вычислите: а) $$\frac{2}{3} + \frac{1}{9}$$; б) $$ \frac{3}{4} : \frac{7}{8}$$; в) $$3 \frac{2}{3} - \frac{2}{7}$$

Решение: а) $$\frac{2}{3} + \frac{1}{9} = \frac{2 \cdot 3}{3 \cdot 3} + \frac{1}{9} = \frac{6}{9} + \frac{1}{9} = \frac{6+1}{9} = \frac{7}{9}$$ б) $$\frac{3}{4} : \frac{7}{8} = \frac{3}{4} \cdot \frac{8}{7} = \frac{3 \cdot 8}{4 \cdot 7} = \frac{3 \cdot 2 \cdot 4}{4 \cdot 7} = \frac{6}{7}$$ в) $$3 \frac{2}{3} - \frac{2}{7} = \frac{3 \cdot 3 + 2}{3} - \frac{2}{7} = \frac{11}{3} - \frac{2}{7} = \frac{11 \cdot 7}{3 \cdot 7} - \frac{2 \cdot 3}{7 \cdot 3} = \frac{77}{21} - \frac{6}{21} = \frac{77-6}{21} = \frac{71}{21} = 3 \frac{8}{21}$$ Ответ: а) $$\frac{7}{9}$$; б) $$\frac{6}{7}$$; в) $$3 \frac{8}{21}$$