3. Вычислите: а) 5/6 + 2/11, б) 15/3 + 5/20, в) 3/7 + 7/15, г) 13/16 - 7/9, д) 14/2 - 21/5, е) 2/9 + 5/12

Решение: а) \(\frac{5}{6} + \frac{2}{11} = \frac{5 \cdot 11 + 2 \cdot 6}{6 \cdot 11} = \frac{55 + 12}{66} = \frac{67}{66}\) б) \(\frac{15}{3} + \frac{5}{20} = 5 + \frac{1}{4} = \frac{20}{4} + \frac{1}{4} = \frac{21}{4}\) в) \(\frac{3}{7} + \frac{7}{15} = \frac{3 \cdot 15 + 7 \cdot 7}{7 \cdot 15} = \frac{45 + 49}{105} = \frac{94}{105}\) г) \(\frac{13}{16} - \frac{7}{9} = \frac{13 \cdot 9 - 7 \cdot 16}{16 \cdot 9} = \frac{117 - 112}{144} = \frac{5}{144}\) д) \(\frac{14}{2} - \frac{21}{5} = 7 - \frac{21}{5} = \frac{35}{5} - \frac{21}{5} = \frac{14}{5}\) е) \(\frac{2}{9} + \frac{5}{12} = \frac{2 \cdot 4 + 5 \cdot 3}{36} = \frac{8 + 15}{36} = \frac{23}{36}\) Ответы: а) 67/66 б) 21/4 в) 94/105 г) 5/144 д) 14/5 е) 23/36