等差数列前n项和为Sn,a1=20,S8=S13,则当Sn取最大值时,n=,
问题描述:
等差数列前n项和为Sn,a1=20,S8=S13,则当Sn取最大值时,n=,
答
S8=8a1+28d
S13=13a1+78d
S8=S13
5a1+50d=0 a1+10d=0
a1=20
d=-2
an=a1+(n-1)d=22-2d
a10=2
a11=0
a12=-2
当Sn取最大值时,n=10或11