数列{an},前n项和Sn=3n(41-n)/2,试求{an},的前30项和.急..

问题描述:

数列{an},前n项和Sn=3n(41-n)/2,试求{an},的前30项和
.
急..

Sn-1=3(n-1)(41-(n-1))/2
题目式减下式得
Sn-Sn-1=3n(41-n)/2-3(n-1)(41-(n-1))/2=-3n-63
Sn-Sn-1=an
所以an=-3n-63
a1=-66
S30=-3285