Question

求{an}的前n项和Sn.(1) a1=-4,a8=-18,n=8 (2)a1=14.5,d=0.7,an=32

Answer 1

(1)是等差数列吗?若是则

S8=8*(a1+a8)/2

=4*(-4-18)

= -88

(2)

an=a1+(n-1)d;代入数值得到

32=14.5+0.7(n-1); 解得

n=26;

所以

S26=26*(a1+a26)/2

=13*(14.5+32)

=604.5