Sn=a×2^n+1,n=1时a1=S1=2a+1,n>1时an=Sn-S=a[2^n-2^(n-1)]=a*2^(n-1),{an}是等比数列，所以a2/a1=a/an,即2a/(2a+1)=2,a=2a+1,a=-1.
