已知{a
n}是公差不为零的等差数列，如果s
n是{a
n}的前n项的和，那么
等于______．
人气：442 ℃ 时间：2020-06-22 22:49:27
解答
设a
n=a
1+（n-1）d，s
n=na
1+
d，代入得
=
=
=2
故答案为2
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