合比性质：若a/b=c/d, 则(a+b)/b=(c+d)/d ; (a-b)/b=(c-d)/d
合比性质的推导：若a/b=c/d a/b=c/d
a/b+1=c/d+1 a/b-1=c/d-1
(a+b)/b=(c+d)/d (a-b)/b=(c-d)/d