Вычислите: a) (2/5 - 1/4) + 9/20 б) 7/30 + (3/5 - 1/6) в) 7/8 - (1/9 + 2/3) г) (5/14 + 9/10) - 5/7

Решение:

1. \( \frac{2}{5} - \frac{1}{4} + \frac{9}{20} = \frac{8}{20} - \frac{5}{20} + \frac{9}{20} = \frac{3 + 9}{20} = \frac{12}{20} = \frac{3}{5} \)


2. \( \frac{7}{30} + \frac{3}{5} - \frac{1}{6} = \frac{7}{30} + \frac{18}{30} - \frac{5}{30} = \frac{7 + 18 - 5}{30} = \frac{20}{30} = \frac{2}{3} \)


3. \( \frac{7}{8} - (\frac{1}{9} + \frac{2}{3}) = \frac{7}{8} - (\frac{1}{9} + \frac{6}{9}) = \frac{7}{8} - \frac{7}{9} = \frac{63 - 56}{72} = \frac{7}{72} \)


4. \( (\frac{5}{14} + \frac{9}{10}) - \frac{5}{7} = (\frac{25}{70} + \frac{63}{70}) - \frac{50}{70} = \frac{88}{70} - \frac{50}{70} = \frac{38}{70} = \frac{19}{35} \)

Ответ: а) 3/5; б) 2/3; в) 7/72; г) 19/35.