3. Вычислите (сложение и вычитание): 2 4 5 1 a) 7+3:6) 18-9; B) 16+24.

Ответ: a) 13/21; б) 1/6; в) 25/12

a) \(\frac{2}{7} + \frac{1}{3} = \frac{2 \cdot 3}{7 \cdot 3} + \frac{1 \cdot 7}{3 \cdot 7} = \frac{6}{21} + \frac{7}{21} = \frac{6+7}{21} = \frac{13}{21}\)

б) \(\frac{13}{18} - \frac{4}{9} = \frac{13}{18} - \frac{4 \cdot 2}{9 \cdot 2} = \frac{13}{18} - \frac{8}{18} = \frac{13-8}{18} = \frac{5}{18}\)

в) \(1 \frac{5}{6} + 2 \frac{1}{4} = \frac{11}{6} + \frac{9}{4} = \frac{11 \cdot 2}{6 \cdot 2} + \frac{9 \cdot 3}{4 \cdot 3} = \frac{22}{12} + \frac{27}{12} = \frac{22+27}{12} = \frac{49}{12} = 4 \frac{1}{12}\)

Ответ: a) 13/21; б) 5/18; в) 49/12