Найди значение выражения: a) 12 2/5 - 2 2/7 : 1 19/21

- a) Приведем смешанные дроби к неправильным: $12\frac{2}{5} = \frac{12*5 + 2}{5} = \frac{62}{5}$, $2\frac{2}{7} = \frac{2*7 + 2}{7} = \frac{16}{7}$, $1\frac{19}{21} = \frac{1*21 + 19}{21} = \frac{40}{21}$. Выполним действия: $$12\frac{2}{5} - 2\frac{2}{7} : 1\frac{19}{21} = \frac{62}{5} - \frac{16}{7} : \frac{40}{21} = \frac{62}{5} - \frac{16}{7} * \frac{21}{40} = \frac{62}{5} - \frac{16 * 21}{7 * 40} = \frac{62}{5} - \frac{2 * 3}{1 * 5} = \frac{62}{5} - \frac{6}{5} = \frac{56}{5} = 11\frac{1}{5}$$ - б) $$ (12\frac{2}{5} - 2\frac{2}{7}) : 1\frac{19}{21} = (\frac{62}{5} - \frac{16}{7}) : \frac{40}{21} = (\frac{62 * 7}{5 * 7} - \frac{16 * 5}{7 * 5}) : \frac{40}{21} = (\frac{434}{35} - \frac{80}{35}) : \frac{40}{21} = \frac{354}{35} : \frac{40}{21} = \frac{354}{35} * \frac{21}{40} = \frac{354 * 21}{35 * 40} = \frac{1866}{2000} = \frac{1866:2}{2000:2} = \frac{933}{1000} = 0,933$$ **Ответ:** - a) $11\frac{1}{5}$ - б) $0,933$