设公差数列首项a,公差d,d≠0
(a+2d)^2=(a+d)(a+5d)
a^2+4ad+4d^2=a^2+6ad+5d^2
d^2=-2ad
d=-2a
等比数列公比：
(a+2d)/(a+d)
=(a-4a)/(a-2a)
=(-3a)/(-a)
=3