2012年高考数学二轮复习同步练习:
专题4 数列
1.(2011•大纲全国卷文,17)设等比数列{an}的前n项和为Sn,已知a3=6,6a1+a3=30,求an和Sn.
[解析] 设{an}的公比为q,由已知有:
a1q=66a1+a1q2=30.解得a1=3q=2或a1=2q=3
(1)当a1=3,q=2时,an=a1•qn-1=3×2n-1
Sn=a11-qn1-q=3×1-2n1-2=3×(2n-1)
(2)当a1=2,q=3时,an=a1•qn-1=2×3n-1
Sn=a11-qn1-q=2×1-3n1-3=3n-1.
综上,an=3×2n-1,Sn=3×(2n-1)或an=2×3n-1,Sn=3n-1.