Вычислите: a) 3 4 - 2 7; б) 1 2 + 3 7; в) 2 9 + 3 7; г) 8 9 - 1 2.

а)
\( \frac{3}{4} - \frac{2}{7} = \frac{21}{28} - \frac{8}{28} = \frac{13}{28} \)

б)
\( \frac{1}{2} + \frac{3}{7} = \frac{7}{14} + \frac{6}{14} = \frac{13}{14} \)

в)
\( \frac{2}{9} + \frac{3}{7} = \frac{14}{63} + \frac{27}{63} = \frac{41}{63} \)

г)
\( \frac{8}{9} - \frac{1}{2} = \frac{16}{18} - \frac{9}{18} = \frac{7}{18} \)