已知递减的等差数列{an}满足a12=a92,则a5=()A.-1B.0C.-1或0D.4或5-数学

题目简介

已知递减的等差数列{an}满足a12=a92,则a5=()A.-1B.0C.-1或0D.4或5-数学

题目详情

已知递减的等差数列{an}满足a12=a92,则a5=(  )
A.-1B.0C.-1或0D.4或5
题型:单选题难度:中档来源:不详

答案

已知递减的等差数列{an}满足a12=a92,则 (a9+a1)(a9-a1)=0,
故有 a9=-a1,∴a5 =
a1+9
2
=0,
故选 B.

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