Question

已知{an}为等差数列,若a11/a10

Answer 1

由已知得,a1＜0,d＞0,a10＜0,a11＞0,

∴a1+a19＜0,a10+a11＞0,

∴a1+a20＞0,

∴S19＜0,S20＞0,

故n=20．