已知Sn为等比数列an的前n项和,若S3=7,S6=63,则a4的值是?
人气:487 ℃ 时间:2020-04-23 12:23:35
解答
解由S3=a1(1-q^3)/(1-q)=7
S6=a1(1-q^6)/(1-q)=63
解得a1=1,q=2
即an=1*(2)^(n-1)
即a4=1*(2)^(4-1)=8.
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