a1=S1=a-1/2
a2=S2-a1=(a-1/4)-(a-1/2)=1/4
a3=S3-S2=(a-1/8)-(a-1/4)=1/8
成等比数列,则a2^2=a1a3
即1/16=1/8(a-1/2)
得a=1