2. a) 2/5 + (-2/15); б) 2/5 + 3/4; в) 11/27 + 5/18

a) \(\frac{2}{5} + \left(-\frac{2}{15}\right) = \frac{2\cdot3}{5\cdot3} - \frac{2}{15} = \frac{6}{15} - \frac{2}{15} = \frac{4}{15}\) б) \(\frac{2}{5} + \frac{3}{4} = \frac{2\cdot4}{5\cdot4} + \frac{3\cdot5}{4\cdot5} = \frac{8}{20} + \frac{15}{20} = \frac{23}{20}\) в) \(\frac{11}{27} + \frac{5}{18} = \frac{11\cdot2}{27\cdot2} + \frac{5\cdot3}{18\cdot3} = \frac{22}{54} + \frac{15}{54} = \frac{37}{54}\)