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数学
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设数列an的前n项和为Sn,若Sn=a1(3的n次方-1)/2(对于所有的n大于等于1）,且a4=54
人气：493 ℃ 时间：2019-08-19 09:01:43
解答
S4=a1*（3^4-1）/2=40a1
S3=a1*(3^3-1)/2=13a1
S4-S3=27a1=a4=54 （对于任何数列,有an=Sn-Sn-1）
∴a1=2
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