31. Вычислите: а) \(\frac{3}{4}-\frac{1}{5}\); б) \(\frac{2}{3}-\frac{4}{7}\); в) \(\frac{1}{11}+\frac{1}{2}\); г) \(\frac{1}{2}+\frac{8}{13}\).

1. а) \(\frac{3}{4}-\frac{1}{5}=\frac{15}{20}-\frac{4}{20}=\frac{11}{20}\).
2. б) \(\frac{2}{3}-\frac{4}{7}=\frac{14}{21}-\frac{12}{21}=\frac{2}{21}\).
3. в) \(\frac{1}{11}+\frac{1}{2}=\frac{2}{22}+\frac{11}{22}=\frac{13}{22}\).
4. г) \(\frac{1}{2}+\frac{8}{13}=\frac{13}{26}+\frac{16}{26}=\frac{29}{26}=1\frac{3}{26}\).

Ответ: а) \(\frac{11}{20}\); б) \(\frac{2}{21}\); в) \(\frac{13}{22}\); г) \(1\frac{3}{26}\).