(1)
a1=S1=3+2a1
a1=-3
S2=a1+a2=3+2a2
a2=a1-3=-3-3=-6
S3=a1+a2+a3=3+2a3
a3=a1+a2-3=-3-6-3=-12
S4=a1+a2+a3+a4=3+2a4
a4=a1+a2+a3-3=-3-6-12-3=-24
(2)
n≥2时,an=Sn-S(n-1)=3+2an-[3+2a(n-1)]
an=2a(n-1)
an/a(n-1)=2,为定值
又a1=-3,数列{an}是以-3为首项,2为公的等比数列.
an=-3×2^(n-1)
数列{an}的通项公式为an=-3×2^(n-1)