Bonjour
2/8 + 5/6 = (2 x 3) / (8 x 3) + (5 x 4) / (6 x 4) = 6/24 + 20/24 = 26/24 = (2 x 13) / (2 x 12) = 13/12.
7/9 - 5/12 = (7 x 4) / (9 x 4) - (5 x 3) / (12 x 3) = 28/36 - 15/36 = 13/36
8/21 - 3/14 = (8 x 2) / (21 x 2) - (3 x 3) / (14 x 3) = 16/42 - 9/42 = 7/42 = (7 x 1) / (7 x 6) = 1/6