问题
选择题
设数列{an}的通项an=(-1)n-1•n,前n项和为Sn,则S2010=( )
A.-2010
B.-1005
C.2010
D.1005
答案
S2010=1+(-2)+3+(-4)+…+2009+(-2010)
=[1+(-2)]+[3+(-4)]+…+[2009+(-2010)]
=(-1)•
=-1005.2010 2
故选B.
设数列{an}的通项an=(-1)n-1•n,前n项和为Sn,则S2010=( )
A.-2010
B.-1005
C.2010
D.1005
S2010=1+(-2)+3+(-4)+…+2009+(-2010)
=[1+(-2)]+[3+(-4)]+…+[2009+(-2010)]
=(-1)•
=-1005.2010 2
故选B.