设公差为d
(1/2)^(a1+a2+a3)=1/8
a1+a2+a3=3
a2=1
b2=1/2
b1+b2+b3=21/8
(1/2)^(1-d)+(1/2)^(1+d)=17/8
解得d＝2,a1=-1
∴an=2n-3