(1/2)^a1+(1/2)^a2+(1/2)a^3=21/8(1/2)^a1*(1/2)^a2*(1/2)^a3=1/8(1/2)^(a1+a2+a3)=1/8a2=1a1=1或a1=4a3=4或1q=2或1/2所以{bn}是等比数列