a/b=c/d两边都加上1,得(a/b)+1=(c/d)+1(a/b)+(b/b)=(c/d)+(d/d)(a+b)/b=(c+d)/da/b=c/d两边都减去1,得(a/b)-1=(c/d)-1(a/b)-(b/b)=(c/d)-(d/d)(a-b)/b=(c-d)/d综合,可得：若a/b=c/d=k,则（a±b）/b=(c±d)/d=k....