(a+b)/(a-b)=(c+d)/(c-d)
变形(a-b+2b)/(a-b)=(c-d+2d)/(c-d)
得出1-2b/(a-b)=1-2d/(c-d)
化简 b/(a-b)=d/(c-d)分子分母同时颠倒得（a-b)/b=(c-d)/d
再化简（或用公式）得 a/b=c/d