а) \( \frac{1}{4} + \frac{1}{6} = \frac{3}{12} + \frac{2}{12} = \frac{5}{12}\); б) \( \frac{1}{3} + \frac{1}{5} = \frac{5}{15} + \frac{3}{15} = \frac{8}{15}\); в) \( \frac{5}{9} + \frac{1}{6} = \frac{30}{54} + \frac{9}{54} = \frac{39}{54} = \frac{13}{18}\).