Question

等差数列{an}的前n项和为Sn,已知S9＜0,S10＞0,则此等差数列的前n项和中,n是多少时取得最小值.

Answer 1

Sn=n*(a1+an)/2

S9=9*(a1+a1+8d)/20

d>0,a1a1>-4.5d

a1=-4d,an=-4d+(n-1)d=0,n=5

a1=-4.5d,an=-4.5d+(n-1)d=0,n=5.5

可知5