4.208 Найдите сумму: 4 a) 元 7 1 1 3 + + 2 ; 7 B)-+ 9 4 )- 7 5 12 4 )-+ e) + 2 ; 3 15 7 10

Ответ: a) -2/7; б) 1/21; в) -2/9; г) -2/21; д) 1/12; е) 1/30

1. a) \[\frac{4}{7} + (-\frac{6}{7}) = \frac{4}{7} - \frac{6}{7} = \frac{4-6}{7} = \frac{-2}{7} = -\frac{2}{7}\]
2. б) \[\frac{1}{3} + (-\frac{2}{7}) = \frac{1}{3} - \frac{2}{7} = \frac{1 \cdot 7}{3 \cdot 7} - \frac{2 \cdot 3}{7 \cdot 3} = \frac{7}{21} - \frac{6}{21} = \frac{7-6}{21} = \frac{1}{21}\]
3. в) \[-\frac{4}{9} + \frac{2}{3} = -\frac{4}{9} + \frac{2 \cdot 3}{3 \cdot 3} = -\frac{4}{9} + \frac{6}{9} = \frac{-4+6}{9} = \frac{2}{9}\]
4. г) \[-\frac{4}{7} + \frac{2}{3} = -\frac{4 \cdot 3}{7 \cdot 3} + \frac{2 \cdot 7}{3 \cdot 7} = -\frac{12}{21} + \frac{14}{21} = \frac{-12+14}{21} = \frac{2}{21}\]
5. д) \[-\frac{5}{12} + \frac{7}{9} = -\frac{5 \cdot 3}{12 \cdot 3} + \frac{7 \cdot 4}{9 \cdot 4} = -\frac{15}{36} + \frac{28}{36} = \frac{-15+28}{36} = \frac{13}{36}\]
6. е) \[\frac{8}{15} + (-\frac{7}{10}) = \frac{8}{15} - \frac{7}{10} = \frac{8 \cdot 2}{15 \cdot 2} - \frac{7 \cdot 3}{10 \cdot 3} = \frac{16}{30} - \frac{21}{30} = \frac{16-21}{30} = \frac{-5}{30} = -\frac{1}{6}\]

Ответ: a) -2/7; б) 1/21; в) -2/9; г) -2/21; д) 1/12; е) 1/30