1. а) 3/5 + 1/5 = 4/5
б) 2/7 + 1/3 = 6/21 + 7/21 = 13/21
в) 3/4 + 5/12 = 9/12 + 5/12 = 14/12 = 1 1/6
д) 2/25 + 1/20 = 8/100 + 10/100 = 18/100 = 9/50
е) 3/7 - 2/7 = 1/7
ж) 11/12 - 2/3 = 11/12 - 8/12 = 3/12 = 1/4
з) 5/21 - 3/14 = 10/42 - 9/42
2. a) 2/15 + (1/15 + 1/3) = 2/15 + (1/15 + 5/15) = 2/15 + 6/15 = 8/15
б) 1/20 + 2/15 + 3/20 + 1/15 = (1/20 + 3/20) + (2/15 + 1/15) = 4/20 + 3/15 = 1/5 + 1/5 = 2/5
3. 1/13 = 1/14 + 1/182