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a1=S1=3 a2=S2-S1=7-3=4 a3=S3-S2=13-7=6 an=Sn-S=[n^2+n+1]-[(n-1)^2+(n-1)+1] =2n {an}的通項公式是:a1=3,an=2n(n=2,3,……) 數列{an}不是等差數列, 但除去第一項後,其餘項按序組成的數列是等差數列
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