Пошаговое объяснение:
12.
(7/8 - 1/8) + 1/4 = 6/8 + 1/4 = 3/4 + 1/4 = 4/4 = 1,
(5/6 + 7/6) - 1/7 = 12/6 - 1/7 = 2 - 1/7 = 1 7/7 - 1/7 = 1 6/7,
(7/3 - 4/3) - 12/13 = 3/3 - 12/13 = 1 - 12/13 = 13/13 - 12/13 = 1/13,
4/3 - (9/11 + 2/11) = 1 1/3 - 11/11 = 1 1/3 - 1 = 1/3,
13.
6 2/7 - 4 1/7 = 2 1/7,
12 1/9 - 5 4/9 = 11 10/9 - 5 4/9 = 6 6/9 = 6 2/3,
8 4/5 - 3 1/5 = 5 3/5,
18 2/11 - 7 5/11 = 17 13/11 - 7 5/11 = 10 8/11