Sn是等比数列{an}的前n项和,且Sn=2^n+K,则实数K=?
人气:489 ℃ 时间:2020-04-06 10:03:51
解答
a1=S1=2+k
a2=S2-S1=(4+k)-(2+k)=2
a3=S3-S2=(8+k)-(4+k)=4
等比则a2²=a1a3
4=4(2+k)
k=-1
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