5.445 Вычислите: а) 1/2 + 1/3; б) 1/3 + 2/7; в) 2/5 + 1/3; г) 3/7 + 4/9; д) 5/9 - 1/6; е) 3/4 - 1/3; ж) 1/6 + 1/3; з) 9/10 - 7/10; и) 1/2 - 3/8; к) 7/15 - 3/10; л) 3/8 + 5/12; м) 5/9 - 1/6; н) 5/11 + 3/5; о) 17/30 - 3/6; п) 17/35 - 4/15

**Ответ:** а) $\frac{5}{6}$ б) $\frac{13}{21}$ в) $\frac{11}{15}$ г) $\frac{55}{63}$ д) $\frac{7}{18}$ е) $\frac{5}{12}$ ж) $\frac{1}{2}$ з) $\frac{1}{5}$ и) $\frac{1}{8}$ к) $\frac{1}{6}$ л) $\frac{19}{24}$ м) $\frac{7}{18}$ н) $\frac{58}{55}$ о) $\frac{1}{15}$ п) $\frac{23}{105}$ **Решение:** а) $\frac{1}{2} + \frac{1}{3} = \frac{1 \cdot 3 + 1 \cdot 2}{6} = \frac{3 + 2}{6} = \frac{5}{6}$ б) $\frac{1}{3} + \frac{2}{7} = \frac{1 \cdot 7 + 2 \cdot 3}{21} = \frac{7 + 6}{21} = \frac{13}{21}$ в) $\frac{2}{5} + \frac{1}{3} = \frac{2 \cdot 3 + 1 \cdot 5}{15} = \frac{6 + 5}{15} = \frac{11}{15}$ г) $\frac{3}{7} + \frac{4}{9} = \frac{3 \cdot 9 + 4 \cdot 7}{63} = \frac{27 + 28}{63} = \frac{55}{63}$ д) $\frac{5}{9} - \frac{1}{6} = \frac{5 \cdot 2 - 1 \cdot 3}{18} = \frac{10 - 3}{18} = \frac{7}{18}$ е) $\frac{3}{4} - \frac{1}{3} = \frac{3 \cdot 3 - 1 \cdot 4}{12} = \frac{9 - 4}{12} = \frac{5}{12}$ ж) $\frac{1}{6} + \frac{1}{3} = \frac{1 + 1 \cdot 2}{6} = \frac{3}{6} = \frac{1}{2}$ з) $\frac{9}{10} - \frac{7}{10} = \frac{2}{10} = \frac{1}{5}$ и) $\frac{1}{2} - \frac{3}{8} = \frac{1 \cdot 4 - 3}{8} = \frac{4 - 3}{8} = \frac{1}{8}$ к) $\frac{7}{15} - \frac{3}{10} = \frac{7 \cdot 2 - 3 \cdot 3}{30} = \frac{14 - 9}{30} = \frac{5}{30} = \frac{1}{6}$ л) $\frac{3}{8} + \frac{5}{12} = \frac{3 \cdot 3 + 5 \cdot 2}{24} = \frac{9 + 10}{24} = \frac{19}{24}$ м) $\frac{5}{9} - \frac{1}{6} = \frac{5 \cdot 2 - 1 \cdot 3}{18} = \frac{10 - 3}{18} = \frac{7}{18}$ н) $\frac{5}{11} + \frac{3}{5} = \frac{5 \cdot 5 + 3 \cdot 11}{55} = \frac{25 + 33}{55} = \frac{58}{55}$ о) $\frac{17}{30} - \frac{3}{6} = \frac{17 - 3 \cdot 5}{30} = \frac{17 - 15}{30} = \frac{2}{30} = \frac{1}{15}$ п) $\frac{17}{35} - \frac{4}{15} = \frac{17 \cdot 3 - 4 \cdot 7}{105} = \frac{51 - 28}{105} = \frac{23}{105}$