Sn=a·2^n+b
a1=S1=2a+b=3
a2=S2-S1=4a+b-(2a+b)=2a
a3=S3-S2=(8a+b)-(4a+b)=4a
等比数列{an}
a1*a3=a2^2
a1=a=3
a2=6
a3=12 2a+b=3 a=3 b=-3