等差数列{an}中,d=2,n=5,an=-10,求a1及Sn

问题描述:

等差数列{an}中,d=2,n=5,an=-10,求a1及Sn

a5=a1+4d
-10=a1+8
a1=-18
S(5)=(a1+a5)*5/2
=(-18-10)*5/2
=-70