50. Найдите сумму: 1) 12 + 8; 2) 13/15 + 6/10; 3) 9/21 + 4/14; 4) 8/9 + 7/12.

Решение:

1. \(12 + 8 = 20\)
2. \(\frac{13}{15} + \frac{6}{10} = \frac{13 \cdot 2}{30} + \frac{6 \cdot 3}{30} = \frac{26 + 18}{30} = \frac{44}{30} = \frac{22}{15} = 1\frac{7}{15}\)
3. \(\frac{9}{21} + \frac{4}{14} = \frac{3}{7} + \frac{2}{7} = \frac{3+2}{7} = \frac{5}{7}\)
4. \(\frac{8}{9} + \frac{7}{12} = \frac{8 \cdot 4}{36} + \frac{7 \cdot 3}{36} = \frac{32 + 21}{36} = \frac{53}{36} = 1\frac{17}{36}\)

Ответ: 1) 20; 2) \(1\frac{7}{15}\); 3) \(\frac{5}{7}\); 4) \(1\frac{17}{36}\).