uw0008
幼苗
共回答了19个问题采纳率:84.2% 举报
(Ⅰ)设等差数列的公差为d,等比数列的公比为q,
则a1=1,a2=2,a3=1+d,a4=2q,a5=1+2d,
∵S3=a4,a3+a5=a4+2.
∴
4+d=2q
(1+d)+(1+2d)=2+2q,
解得d=2,q=3,
∴an=
n,n=2k−1
2•3
n
2−1,n=2k.
(Ⅱ)当n=1时,S2n=2n+n2.
当n≥2时,
S2n=
(1+2n−1)n
2+
2(1−3n)
1−3
=n2-1+3n=n2-1+(1+2)n>n2-1+2n+1>2n+n2.
1年前
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