a2,a3,a6成等比,则
a3^2=a2a6
(a2+d)^2=a2(a2+4d)
整理,得
d(d-2a2)=0
d不等于0,要等式成立,则2a2=d a2=d/2
a3=a2+d=d/2 +d=(3/2)d
a3/a2=(3/2)d/(d/2)=3
公比是3.