等比则a3²=a2a6
(a1+2d)²=(a1+d)(a1+5d)
a1²+4a1d+4d²=a1²+6a1d+5d²
2a1d+d²=0
d≠0
d=-2a1
所以q=a3/a2
=(a1+2d)/(a1+3d)
=-3a1/(-5a1)
=3/5