a/b=c/d ad=bc(a+c)/(a-c)-(b+d)/(b-d)=[(a+c)(b-d)-(b+d)(a-c)]/(a-c)(b-d)=[ab-ad+bc-cd-ab+bc-ad+cd]/(a-c)(b-d)=(2bc-2ad)/(a-c)(b-d)=0(a+c)/(a-c)=(b+d)/(b-d)
