-5/13+(-7/13)=-5/13-7/13=-12/13
3/11+(-8/11)=3/11-8/11=-5/11
-9/17+13/17=4/17
-2/5+(-2/15)=-2/5-2/15=-6/15-2/15=-8/15
-2/5+3/4=-8/20+15/20=7/20
-11/24+5/18=-22/54+15/54=-7/54
-3/11+(-7/11)=-3/11-7/11=-10/11
6/13+(-8/13)=-2/13
-8/17+11/17=3/17
-1/4+(-3/16)=-1/4-3/16=-4/16-3/16=-7/16
-3/4+2/5=-15/20+8/20=-7/20
-5/36+5/27=-15/108+20/108=5/108