求数列的前n项和:
1+1,+4,+7,…,+3n−2,….
人气:331 ℃ 时间:2019-11-23 10:20:36
解答
设
Sn=(1+1)+(+4)+(+7)+…+(+3n−2)将其每一项拆开再重新组合得
Sn=(1+++…+)+(1+4+7+…+3n−2)当a=1时,
Sn=n+=
当a≠1时,
Sn=+=
+
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