数列{an}满足an+an+1=1/2(n∈N*),a1=−1/2,Sn是{an}的前n项和,则S2011=_.
问题描述:
数列{an}满足an+an+1=
(n∈N*),a1=−1 2
,Sn是{an}的前n项和,则S2011=______. 1 2
答
∵an+an+1=
(n∈N*),a1=−1 2
,1 2
S2011=a1+(a2+a3)+(a4+a5)+…+(a2010+a2011)
=-
+1 2
+…+1 2
1 2
=−
+1 2
×10051 2
=502
故答案为:502