等比数列{an}的前n项和Sn=3^n+a,则a2S2等于?
人气:440 ℃ 时间:2020-06-03 19:45:10
解答
an=Sn-S(n-1)=3^n-3^(n-1)=2*3^(n-1)
a1=S1=3+a=2
a=-1
an=2*3^(n-1)
a2=6
Sn=3^n-1
S2=8
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