19. Вычислите: 1) 3/5 + (−2/3); 2) −3/7 + 3/5; 3) 5/12 + (−3/10); 4) −7/8 + 1 5/6; 5) −1 1/6 + 1 5/18.

1. \(\frac{3}{5}+\left(−\frac{2}{3}\right)=\frac{9}{15}−\frac{10}{15}=−\frac{1}{15}\)
2. \(−\frac{3}{7}+\frac{3}{5}=−\frac{15}{35}+\frac{21}{35}=\frac{6}{35}\)
3. \(\frac{5}{12}+\left(−\frac{3}{10}\right)=\frac{25}{60}−\frac{18}{60}=\frac{7}{60}\)
4. \(−\frac{7}{8}+1\frac{5}{6}=−\frac{21}{24}+\frac{44}{24}=\frac{23}{24}\)
5. \(−1\frac{1}{6}+1\frac{5}{18}=−\frac{21}{18}+\frac{23}{18}=\frac{2}{18}=\frac{1}{9}\)

Ответ: 1) \(−\frac{1}{15}\); 2) \(\frac{6}{35}\); 3) \(\frac{7}{60}\); 4) \(\frac{23}{24}\); 5) \(\frac{1}{9}\).