1/(2007*2008)+1/(2008*2009)+1/(2010*2011)+2011
=1/2007-1/2008+1/2008-1/2009+1/2009-1/1010+1/2010-1/2011+2011
=1/2007-1/2011+2011
1/(2007*2008)+1/(2008*2009)+1/(2010*2011)+2011
=1/2007-1/2008+1/2008-1/2009+1/2009-1/1010+1/2010-1/2011+2011
=1/2007-1/2011+2011