数列{an}的前n项和Sn=1/2n²-3/2n,求通项an,求a4,求a3+...+a8

数列{an}的前n项和Sn=1/2n²-3/2n,求通项an,求a4,求a3+...+a8
数列{an}的前n项和Sn=1/2n²-3/2n,求通项an,求a4,求a3+...+a8

数列{an}的前n项和Sn=1/2n²-3/2n,求通项an,求a4,求a3+...+a8
a1=S1=﹣1
an=Sn-S﹙n-1﹚(n≥2﹚
=n-2
∴a1适合an=n-2(n≥2﹚
∴an=n-2
a4=2,
a3+...+a8
=1+2+……+6=21