a:b=c:d ==> ad = bc
要想判断(a+b):(a-b）=（c+d):(c-d）是否成立
只要下面两式子相等：
(a+b)(c-d)=ac+bc-ad-bd .(1)
(a-b)(c+d)=ac-bc+ad-bd .(2)
a:b=c:d
==>ad = bc
==>式(1)=式(2)
即(a+b)(c-d)=(a-b)(c+d)
==>(a+b):(a-b）=（c+d):(c-d)
得证