Question

已知等数列an中,Sn是an前n项和,S3=15,a5=-1

Answer 1

1.是等差数列吗?

S3=3a1+3d=15

a5=a1+4d=-1

a1=7,d=-2

an=a1+(n-1)d=9-2n

Sn=na1+n(n-1)d/2=8n-n^2

2.Sn=8n-n^2=-(n-4)^2+16

n=4时,Sn最大,等于16