等比数列an中a1=4,前n项和Sn满足S3 S2 S4成等差数列,求an通项公式!
人气:295 ℃ 时间:2019-08-20 17:31:05
解答
a1*(1-q^3)/(1-q)+a1*(1-q^4)/(1-q)=2*a1*(1-q^2)/(1-q)
解得:q=0(舍去),q=1,q=-2
q=1时,an=4,
q=-2时,an=4*(-2)^(n-1)
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