a/b=b/c=c/d=d/a

令a/b=b/c=c/d=d/a=k,则a=bk,b=ck,c=dk,d=ak

相加得(a+b+c+d)=(a+b+c+d)k,当(a+b+c+d)≠0时,k=1

所以当(a+b+c+d)=0时(a+b+c+d)/(a+b+c-d)=0

当(a+b+c+d)≠0时,k=1,a=b=c=d≠0,(a+b+c+d)/(a+b+c-d)=4a/2a=2