1) - 3/7 + 2/21 = - 9/21 + 2/21 = - (9-2)/21 = - 7/21 = - 1/3
2) 5/6 + (- 5/7) = 35/42 + (- 30/42) = (35-30)/42 = 5/42
3) - 17/21 + 9 /14 = - 34/42 + 27/42 = - (34-27)/42 = - 7/42 = - 1/6
4)3,9 + (- 3,9) = 3,9 - 3,9 = 0
5)2,77 + (-1/4) = 2,77 - 0,25 = 2,52
6)1/3 + (-1/2) = 2/6 + (-3/6) = - (3-2)/6 = - 1/6
7)2 5/12 + (-17 3/8) = - ( 17 9/24 - 2 10/24) = - 14 23/24
8)-29 + 17 1/8 = - (29 - 17 1/8) = -11 7/8
9)17 + (-1,25) = 17,00 - 1,25 = 15,75
10)-2 3/5 + (-7 3/10) = - (2,6 +7,3) = - 9,9
11)4 3/4 + (-4 11/12) = 4 9/12 + (-4 11/12) = -2/12 = -1/6