a-b=b-c,即a=2b-c,c=2b-a
交换后acb或bac等比
c^2=ab,或a^2=bc
把a=2b-c代入c^2=ab
c^2=(2b-c)b
(c+2b)(c-b)=0
c-b≠0
c+2b=0
b/c=-1/2
同理将c=2b-a代入a^2=bc
(a+2b)(a-b)=0
a/b=-2
公比-1/2,或-2