解:因为A/B=B/C=C/D=D/A,移项,得:A^2=BD=B/C=C/D,再移项,得:A^2=BD=C^2=BD.同理,D^2=AC=B^2=AC.所以,A=±C,B=±D.带入方程,若,A=-C,B=-D,则=0;若A=C,B=D,则=(A-B)/B;若A=C,B=-D,则方程无意义.若A=-C,B=D,则和第一个一样.
