268. Вычислите: 1) 3/7 + 4/9; 2) 8/9 − 7/8; 3) 13/15 − 2/3; 4) 20/21 + 3/7; 5) 17/18 − 11/12; 6) 7/16 + 1/6; 7) 2/9 + 5/6; 8) 10/21 + 9/14; 9) 7/9 − 4/15; 10) 9/14 − 3/7 + 15/28; 11) 1/6 + 1/4 − 1/8; 12) 13/18 − 29/45 + 8/15.

1. \(\frac{3}{7}+\frac{4}{9}=\frac{27}{63}+\frac{28}{63}=\frac{55}{63}\).
2. \(\frac{8}{9}-\frac{7}{8}=\frac{64}{72}-\frac{63}{72}=\frac{1}{72}\).
3. \(\frac{13}{15}-\frac{2}{3}=\frac{13}{15}-\frac{10}{15}=\frac{1}{5}\).
4. \(\frac{20}{21}+\frac{3}{7}=\frac{20}{21}+\frac{9}{21}=\frac{29}{21}=1\frac{8}{21}\).
5. \(\frac{17}{18}-\frac{11}{12}=\frac{34}{36}-\frac{33}{36}=\frac{1}{36}\).
6. \(\frac{7}{16}+\frac{1}{6}=\frac{21}{48}+\frac{8}{48}=\frac{29}{48}\).
7. \(\frac{2}{9}+\frac{5}{6}=\frac{4}{18}+\frac{15}{18}=\frac{19}{18}=1\frac{1}{18}\).
8. \(\frac{10}{21}+\frac{9}{14}=\frac{20}{42}+\frac{27}{42}=\frac{47}{42}=1\frac{5}{42}\).
9. \(\frac{7}{9}-\frac{4}{15}=\frac{35}{45}-\frac{12}{45}=\frac{23}{45}\).
10. \(\frac{9}{14}-\frac{3}{7}+\frac{15}{28}=\frac{18}{28}-\frac{12}{28}+\frac{15}{28}=\frac{21}{28}=\frac{3}{4}\).
11. \(\frac{1}{6}+\frac{1}{4}-\frac{1}{8}=\frac{4}{24}+\frac{6}{24}-\frac{3}{24}=\frac{7}{24}\).
12. \(\frac{13}{18}-\frac{29}{45}+\frac{8}{15}=\frac{65}{90}-\frac{58}{90}+\frac{48}{90}=\frac{55}{90}=\frac{11}{18}\).

Ответ: 1) \(\frac{55}{63}\); 2) \(\frac{1}{72}\); 3) \(\frac{1}{5}\); 4) \(1\frac{8}{21}\); 5) \(\frac{1}{36}\); 6) \(\frac{29}{48}\); 7) \(1\frac{1}{18}\); 8) \(1\frac{5}{42}\); 9) \(\frac{23}{45}\); 10) \(\frac{3}{4}\); 11) \(\frac{7}{24}\); 12) \(\frac{11}{18}\).