Вычислите. 1 вариант: 1) 1/4 + 2/5 =; 2) 5/7 − 1/2 =; 3) 1/3 · 6/10 =; 4) 3/5 : 1/5 =; 5) 2 3/4 + 1 5/6 =; 6) 4 1/4 − 2/3 =; 7) 1 1/2 · 3 4/9 =; 8) 4 1/6 : 1 2/3 =; 9) 1/2 + 2 =; 10) 3 − 2/5 =; 11) 5 · 2/5 =; 12) 7/8 : 7 =.

1. Приводим к общему знаменателю: \(\frac{1}{4}+\frac{2}{5}=\frac{5}{20}+\frac{8}{20}=\frac{13}{20}\).
2. \(\frac{5}{7}-\frac{1}{2}=\frac{10}{14}-\frac{7}{14}=\frac{3}{14}\).
3. \(\frac{1}{3}\cdot\frac{6}{10}=\frac{6}{30}=\frac{1}{5}\).
4. Деление на дробь заменяем умножением на обратную дробь: \(\frac{3}{5}:\frac{1}{5}=\frac{3}{5}\cdot5=3\).
5. \(2\frac{3}{4}=\frac{11}{4}\), \(1\frac{5}{6}=\frac{11}{6}\). Тогда \(\frac{11}{4}+\frac{11}{6}=\frac{33}{12}+\frac{22}{12}=\frac{55}{12}=4\frac{7}{12}\).
6. \(4\frac{1}{4}=\frac{17}{4}\). Тогда \(\frac{17}{4}-\frac{2}{3}=\frac{51}{12}-\frac{8}{12}=\frac{43}{12}=3\frac{7}{12}\).
7. \(1\frac{1}{2}=\frac{3}{2}\), \(3\frac{4}{9}=\frac{31}{9}\). Тогда \(\frac{3}{2}\cdot\frac{31}{9}=\frac{93}{18}=\frac{31}{6}=5\frac{1}{6}\).
8. \(4\frac{1}{6}=\frac{25}{6}\), \(1\frac{2}{3}=\frac{5}{3}\). Тогда \(\frac{25}{6}:\frac{5}{3}=\frac{25}{6}\cdot\frac{3}{5}=\frac{5}{2}=2\frac{1}{2}\).
9. \(\frac{1}{2}+2=2\frac{1}{2}\).
10. \(3-\frac{2}{5}=\frac{15}{5}-\frac{2}{5}=\frac{13}{5}=2\frac{3}{5}\).
11. \(5\cdot\frac{2}{5}=2\).
12. \(\frac{7}{8}:7=\frac{7}{8}\cdot\frac{1}{7}=\frac{1}{8}\).

Ответ: 1) \(\frac{13}{20}\); 2) \(\frac{3}{14}\); 3) \(\frac{1}{5}\); 4) 3; 5) \(4\frac{7}{12}\); 6) \(3\frac{7}{12}\); 7) \(5\frac{1}{6}\); 8) \(2\frac{1}{2}\); 9) \(2\frac{1}{2}\); 10) \(2\frac{3}{5}\); 11) 2; 12) \(\frac{1}{8}\).