考点3 等比数列前n项和及性质的应用(2018全国卷(理)记Sn为数列an的前n项和若Sn2an1,则S6_.【解析】Sn2an1,当n2时,Sn12an11,anSnSn12an2an1(n2),即an2an1(n2)当n1时,a1S12a11,得a11.数列an是首项a11,公比q2的等比数列,Sna11-qn1-q-11-2n1-212n,S612663.【答案】63
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