3. Вычислите: 1) \(\frac{2}{7} + \frac{3}{8}\); 2) \(\frac{5}{6} - \frac{4}{9}\); 3) \(3\frac{1}{8} + 2\frac{5}{6}\); 4) \(5\frac{11}{12} - 3\frac{3}{18}\)

1) \(\frac{2}{7} + \frac{3}{8} = \frac{2 \cdot 8}{7 \cdot 8} + \frac{3 \cdot 7}{8 \cdot 7} = \frac{16}{56} + \frac{21}{56} = \frac{16+21}{56} = \frac{37}{56}\)

2) \(\frac{5}{6} - \frac{4}{9} = \frac{5 \cdot 3}{6 \cdot 3} - \frac{4 \cdot 2}{9 \cdot 2} = \frac{15}{18} - \frac{8}{18} = \frac{15-8}{18} = \frac{7}{18}\)

3) \(3\frac{1}{8} + 2\frac{5}{6} = 3 + \frac{1}{8} + 2 + \frac{5}{6} = (3+2) + (\frac{1}{8} + \frac{5}{6}) = 5 + (\frac{1 \cdot 3}{8 \cdot 3} + \frac{5 \cdot 4}{6 \cdot 4}) = 5 + (\frac{3}{24} + \frac{20}{24}) = 5 + \frac{23}{24} = 5\frac{23}{24}\)

4) \(5\frac{11}{12} - 3\frac{3}{18} = 5\frac{11}{12} - 3\frac{1}{6} = 5 + \frac{11}{12} - 3 - \frac{1}{6} = (5-3) + (\frac{11}{12} - \frac{1}{6}) = 2 + (\frac{11}{12} - \frac{1 \cdot 2}{6 \cdot 2}) = 2 + (\frac{11}{12} - \frac{2}{12}) = 2 + \frac{9}{12} = 2 + \frac{3}{4} = 2\frac{3}{4}\)

Ответ: 1) \(\frac{37}{56}\); 2) \(\frac{7}{18}\); 3) \(5\frac{23}{24}\); 4) \(2\frac{3}{4}\)