n=1时,a1=S1=3+2=5n≥2时,Sn=3n^2+2nSn-1=3(n-1)^2+2(n-1)an=Sn-Sn-1=3n^2+2n-3(n-1)^2-2(n-1)=6n-1n=1时,a1=6-1=5,同样满足.综上,得数列{an}的通项公式为an=6n-1.一楼的错误在于没有判断a1是不是满足通项公式....