2022外研版英语周报60期答案

7.解:(1)由题意得Sn-S=Sn=S1 2(n≥2),即an1-an=2(n≥2),2分又a2-a1=3-1=2,所以an1-an=2,(n21)所以数列{an}是以1为首项,公差为2的等差数列,所以an=2n-15分注:对于结果对但没有考虑n=1的扣2分(2)bn=an 2=(2n-1) 221=(2n-1) x46分所以=[ 3 5 … (21=1) 2×(4 4 4 … 4)2(4-1).10分

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