题目内容
等差数列{an}中,Sn是其前n项和,a1=-2008时,
-
=2,则S2008的值为( )
S2007 |
2007 |
S2005 |
2005 |
A.-2006 | B.2006 | C.-2008 | D.2008 |
因为S2007=2007×(-2008)+
d,
S2005=2005×(-2008)+
d,
则
-
=[2007×(-2008)+
d]-[2005×(-2008)+
d]=2,
化简可得d=2.则S2008=2008×(-2008)+
×2=2008×(-2008+2007)=-2008
故选C
2007×2006 |
2 |
S2005=2005×(-2008)+
2005×2004 |
2 |
则
S2007 |
2007 |
S2005 |
2005 |
2007×2006 |
2 |
2005×2004 |
2 |
化简可得d=2.则S2008=2008×(-2008)+
2008×2007 |
2 |
故选C

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