数列{an}是等差数列,若a11/a10

问题描述:

数列{an}是等差数列,若a11/a10

因为Sn有最大值,所以公差小于0,a11/a10

a11/a10a100
Sn存在最小值与题目条件矛盾,
a10>0时,则a11+a10S19=19(a1+a19)/2=19a10>0
S20=20(a1+a20)/2=10(a10+a11)当Sn取最小正值时,n=19