572. a) $$(\frac{1}{2} + \frac{1}{5}) : 2 - \frac{3}{10}$$; б) $$(\frac{3}{4} - \frac{5}{12}) \cdot 8 + \frac{1}{18}$$; в) $$(\frac{7}{15} + \frac{4}{5}) : 3 - \frac{1}{9}$$; г) $$(\frac{3}{7} - \frac{1}{21}) \cdot 14 - \frac{1}{6}$$

а)

$$(\frac{1}{2} + \frac{1}{5}) : 2 - \frac{3}{10} = \frac{5+2}{10} : 2 - \frac{3}{10} = \frac{7}{10} : 2 - \frac{3}{10} = \frac{7}{10} \cdot \frac{1}{2} - \frac{3}{10} = \frac{7}{20} - \frac{3}{10} = \frac{7}{20} - \frac{6}{20} = \frac{1}{20}$$

б)

$$(\frac{3}{4} - \frac{5}{12}) \cdot 8 + \frac{1}{18} = \frac{9-5}{12} \cdot 8 + \frac{1}{18} = \frac{4}{12} \cdot 8 + \frac{1}{18} = \frac{1}{3} \cdot 8 + \frac{1}{18} = \frac{8}{3} + \frac{1}{18} = \frac{48}{18} + \frac{1}{18} = \frac{49}{18} = 2\frac{13}{18}$$

в)

$$(\frac{7}{15} + \frac{4}{5}) : 3 - \frac{1}{9} = \frac{7+12}{15} : 3 - \frac{1}{9} = \frac{19}{15} : 3 - \frac{1}{9} = \frac{19}{15} \cdot \frac{1}{3} - \frac{1}{9} = \frac{19}{45} - \frac{1}{9} = \frac{19}{45} - \frac{5}{45} = \frac{14}{45}$$

г)

$$(\frac{3}{7} - \frac{1}{21}) \cdot 14 - \frac{1}{6} = \frac{9-1}{21} \cdot 14 - \frac{1}{6} = \frac{8}{21} \cdot 14 - \frac{1}{6} = \frac{8}{3} \cdot 2 - \frac{1}{6} = \frac{16}{3} - \frac{1}{6} = \frac{32}{6} - \frac{1}{6} = \frac{31}{6} = 5\frac{1}{6}$$