你这样看行不行？
设等差数列的首项a1，公差d
sn=na1+1/2n(n-1)d
设An=sn/n=a1+1/2（n-1)d
那么An-An-1=d/2
高二数学数列：设sn是等差数列an的前n项和 求证sn/n是等差数列，这道题目我做出来公差是0，求大神详解！
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