S4n=30
Sn=2,S3n=14.因Sn、S2n-Sn、S3n-S2n成等比数列,则:(S2n-2)²=2(14-S2n),
得:S2n=6或S2n=-4【舍去】,从而Sn=2,S2n-Sn=4,S3n-S2n=8,S4n-S3n=16,即:S4n=30
S4n=30
Sn=2,S3n=14.因Sn、S2n-Sn、S3n-S2n成等比数列,则:(S2n-2)²=2(14-S2n),
得:S2n=6或S2n=-4【舍去】,从而Sn=2,S2n-Sn=4,S3n-S2n=8,S4n-S3n=16,即:S4n=30