如果a/b=c/d=…=m/n(b±d±…±n≠0),那么(a±c±…±m)/(b±d±…±n)=a/b
證 明:設a/b=c/d=…=m/n = k
則a = bk, c = dk,…m = nk
則(a+c+…+m)/(b+d+…+n) = (bk + dk +...+ nk)/(b+d+…+n) = k = a/b
合比性質:如果a/b=c/d,那么(a±b)/b=(c±d)/d,a/(b±a)=c/(d±c)
證 明:當b≠0且d≠0時
a/b=c/d
a/b+1=c/d+1
a/b+b/b=c/d+d/d
(a+b)/b=(c+d)/d