Question

已知各项均为正数的等差数列{An},满足An,Sn,An的平方 成等差数列 求S100

Answer 1

可用递推法：2Sn=An+An*An 递推 2Sn-1=An-1 +An-1*An-1

两市相减,得：An+An-1=An*An-An-1*An-1

因为An为正数,所以 An-An-1=1

之后求An,然后用求和公式算Sn,求得S100