287. Выполните действия: a) 3+85; 12' 6) 100 - 3; r) 81-4; e) 70-20: -

a) Представим число 3 в виде дроби со знаменателем 12: $$3 = \frac{3 \cdot 12}{12} = \frac{36}{12}$$. Тогда:

$$3 + 8\frac{5}{12} = \frac{36}{12} + 8\frac{5}{12} = 8\frac{36+5}{12} = 8\frac{41}{12} = 8 + \frac{41}{12} = 8 + \frac{36 + 5}{12} = 8 + \frac{36}{12} + \frac{5}{12} = 8 + 3 + \frac{5}{12} = 11\frac{5}{12}$$.

б) $$10\frac{6}{7} - 3 = (10 - 3) + \frac{6}{7} = 7\frac{6}{7}$$.

г) $$8\frac{7}{12} - 4\frac{2}{12} = (8 - 4) + \frac{7-2}{12} = 4 + \frac{5}{12} = 4\frac{5}{12}$$.

e) $$7\frac{5}{9} - 2\frac{8}{9} = (7 - 2) + \frac{5}{9} - \frac{8}{9} = 5 + \frac{5}{9} - \frac{8}{9} = 5 - \frac{3}{9} = 4 + \frac{9}{9} - \frac{3}{9} = 4 + \frac{6}{9} = 4\frac{6}{9} = 4\frac{2}{3}$$.

Ответ: а) $$11\frac{5}{12}$$; б) $$7\frac{6}{7}$$; г) $$4\frac{5}{12}$$; е) $$4\frac{2}{3}$$