a: =-3/17+2/3+3/17=2/3
b: =-1/6+5/12+7/12=1-1/6=5/6
c: =(-5/7-2/7)+(3/4+1/4)-1/5=-1/5
d: =(-3/31-28/31)+(-6/17-11/17)+1/25-4/5=-2+1/25-20/25=-69/25
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