№3. 1. \frac{1}{5} + \frac{2}{5} 2. \frac{1}{4} + \frac{2}{3} 3. \frac{1}{3} + \frac{1}{6} + \frac{1}{18} 4. \frac{5}{6} - \frac{2}{3} 5. 1 - \frac{3}{7}

Номер 3

1. \(\frac{1}{5} + \frac{2}{5} = \frac{1+2}{5} = \frac{3}{5}\)
2. \(\frac{1}{4} + \frac{2}{3} = \frac{1 \cdot 3}{4 \cdot 3} + \frac{2 \cdot 4}{3 \cdot 4} = \frac{3}{12} + \frac{8}{12} = \frac{3+8}{12} = \frac{11}{12}\)
3. \(\frac{1}{3} + \frac{1}{6} + \frac{1}{18} = \frac{1 \cdot 6}{3 \cdot 6} + \frac{1 \cdot 3}{6 \cdot 3} + \frac{1}{18} = \frac{6}{18} + \frac{3}{18} + \frac{1}{18} = \frac{6+3+1}{18} = \frac{10}{18} = \frac{5}{9}\)
4. \(\frac{5}{6} - \frac{2}{3} = \frac{5}{6} - \frac{2 \cdot 2}{3 \cdot 2} = \frac{5}{6} - \frac{4}{6} = \frac{5-4}{6} = \frac{1}{6}\)
5. \(1 - \frac{3}{7} = \frac{7}{7} - \frac{3}{7} = \frac{7-3}{7} = \frac{4}{7}\)