2.162. Вычислите: а) 1/2 + 1/5; б) 1/21 + 1/7; в) 3/5 + 2/17; г) 1/7 + 7/9; д) 5/7 + 0; е) 2/3 − 2/5; ж) 1/2 − 1/13; з) 3/5 − 4/7; и) 5/7 − 2/9; к) 4/21 − 0; л) 4/7 + 4/5; м) 7/12 + 13/21.

1. \(\frac{1}{2}+\frac{1}{5}=\frac{5}{10}+\frac{2}{10}=\frac{7}{10}\).
2. \(\frac{1}{21}+\frac{1}{7}=\frac{1}{21}+\frac{3}{21}=\frac{4}{21}\).
3. \(\frac{3}{5}+\frac{2}{17}=\frac{51}{85}+\frac{10}{85}=\frac{61}{85}\).
4. \(\frac{1}{7}+\frac{7}{9}=\frac{9}{63}+\frac{49}{63}=\frac{58}{63}\).
5. \(\frac{5}{7}+0=\frac{5}{7}\).
6. \(\frac{2}{3}-\frac{2}{5}=\frac{10}{15}-\frac{6}{15}=\frac{4}{15}\).
7. \(\frac{1}{2}-\frac{1}{13}=\frac{13}{26}-\frac{2}{26}=\frac{11}{26}\).
8. \(\frac{3}{5}-\frac{4}{7}=\frac{21}{35}-\frac{20}{35}=\frac{1}{35}\).
9. \(\frac{5}{7}-\frac{2}{9}=\frac{45}{63}-\frac{14}{63}=\frac{31}{63}\).
10. \(\frac{4}{21}-0=\frac{4}{21}\).
11. \(\frac{4}{7}+\frac{4}{5}=\frac{20}{35}+\frac{28}{35}=\frac{48}{35}=1\frac{13}{35}\).
12. \(\frac{7}{12}+\frac{13}{21}=\frac{49}{84}+\frac{52}{84}=\frac{101}{84}=1\frac{17}{84}\).

Ответ: а) \(\frac{7}{10}\); б) \(\frac{4}{21}\); в) \(\frac{61}{85}\); г) \(\frac{58}{63}\); д) \(\frac{5}{7}\); е) \(\frac{4}{15}\); ж) \(\frac{11}{26}\); з) \(\frac{1}{35}\); и) \(\frac{31}{63}\); к) \(\frac{4}{21}\); л) \(1\frac{13}{35}\); м) \(1\frac{17}{84}\).