1 - 2 + 3 - 4 +...+ 2007 - 2008 + 2009 - 2010 + 2011
= (1 - 2) + (3 - 4) +...+ (2007 - 2008) + (2009 - 2010) + 2011
= (-1)×1005 + 2011
= -1005 + 2011
= 1006
1 - 2 + 3 - 4 +...+ 2007 - 2008 + 2009 - 2010 + 2011
= (1 - 2) + (3 - 4) +...+ (2007 - 2008) + (2009 - 2010) + 2011
= (-1)×1005 + 2011
= -1005 + 2011
= 1006