Вариант 2. Вычислите: 11) 1/4 + 1/12; 12) 1/6 + 2/3; 13) 7/8 + 3/4; 14) 4/7 + 2/21; 15) 9/12 + 3/4; 16) 8/9 + 1/3; 17) 4/5 + 7/15; 18) 2/21 + 5/14; 19) 3/18 + 5/27; 20) 3/15 + 7/20.

Приводим дроби к общему знаменателю и складываем числители.

1. \(\frac{1}{4}+\frac{1}{12}=\frac{3}{12}+\frac{1}{12}=\frac{1}{3}\).
2. \(\frac{1}{6}+\frac{2}{3}=\frac{1}{6}+\frac{4}{6}=\frac{5}{6}\).
3. \(\frac{7}{8}+\frac{3}{4}=\frac{7}{8}+\frac{6}{8}=\frac{13}{8}=1\frac{5}{8}\).
4. \(\frac{4}{7}+\frac{2}{21}=\frac{12}{21}+\frac{2}{21}=\frac{2}{3}\).
5. \(\frac{9}{12}+\frac{3}{4}=\frac{9}{12}+\frac{9}{12}=\frac{18}{12}=1\frac{1}{2}\).
6. \(\frac{8}{9}+\frac{1}{3}=\frac{8}{9}+\frac{3}{9}=\frac{11}{9}=1\frac{2}{9}\).
7. \(\frac{4}{5}+\frac{7}{15}=\frac{12}{15}+\frac{7}{15}=\frac{19}{15}=1\frac{4}{15}\).
8. \(\frac{2}{21}+\frac{5}{14}=\frac{4}{42}+\frac{15}{42}=\frac{19}{42}\).
9. \(\frac{3}{18}+\frac{5}{27}=\frac{9}{54}+\frac{10}{54}=\frac{19}{54}\).
10. \(\frac{3}{15}+\frac{7}{20}=\frac{4}{20}+\frac{7}{20}=\frac{11}{20}\).