由
S 2009
2009 -
S 2007
2007 =2 可得
a 1 + a 2009
2 -
a 1 + a 2007
2 =2,∴公差d=2.
则S 2011 =2011×a 1+
2011(2011-1)
2 ×d =-2011,
故答案为:-2011.
由
S 2009
2009 -
S 2007
2007 =2 可得
a 1 + a 2009
2 -
a 1 + a 2007
2 =2,∴公差d=2.
则S 2011 =2011×a 1+
2011(2011-1)
2 ×d =-2011,
故答案为:-2011.