3/7+2/21；5/6+[-5/7]；-17/21+9/14；-29+17 1/8；-2 3/5+[-7 3/10]；3/9+(-3/9)；2.77+[-1/4]；1/3+[-1/2]；2 5/12+[-17 3/8]；17+[-1.25] ；4 3/4+[-4 11/14]

-3/7+ 2/21 = -9/21 +2/21= - 7/21
5/6 + (-5/7)=35/42+(-30/42)=5/42
-17/21+9/14= - 34/42 + 27/42= - 7/42= -1/6
-29 +17 1/8= - 11 7/8
-2 3/5 + (- 7 3/10)= -2 6/10 +(-7 3/10)= - 9 9/10
3/9 + (-3/9)=0
1/3 + (-1/2)=2/6 +(- 3/6)= - 1/6
2 5/12+ (-17 3/8)=2 10/24+ (- 17 9/24)= - 15 1/24
17 + (- 1,25)=15,75
4 3/4 + (-4 11/14)=4 21/28 +(- 4 22/28)= - 1 /28