1. Выполните действия: a) $$rac{14}{21} + \frac{11}{21} - \frac{8}{21}$$; б) $$7\frac{15}{16} - 2\frac{8}{16} + 5\frac{9}{16}$$; в) $$5\frac{9}{19} - 2\frac{12}{19} + 9\frac{8}{19}$$

Решение:

a) $$\frac{14}{21} + \frac{11}{21} - \frac{8}{21} = \frac{14 + 11 - 8}{21} = \frac{25 - 8}{21} = \frac{17}{21}$$

б) $$7\frac{15}{16} - 2\frac{8}{16} + 5\frac{9}{16} = (7 - 2 + 5) + (\frac{15}{16} - \frac{8}{16} + \frac{9}{16}) = 10 + \frac{15 - 8 + 9}{16} = 10 + \frac{16}{16} = 10 + 1 = 11$$

в) $$5\frac{9}{19} - 2\frac{12}{19} + 9\frac{8}{19} = (5 - 2 + 9) + (\frac{9}{19} - \frac{12}{19} + \frac{8}{19}) = 12 + \frac{9 - 12 + 8}{19} = 12 + \frac{5}{19} = 12\frac{5}{19}$$