a-b
=2007/2008-2008/2009
=(2007*2009-2008²)/2008*2009
=[(2008-1)(2008+1)-2008²]/2008*2009
=(2008²-1-2008²)/2008*2009
=-1/2008*2009
a
a-b
=2007/2008-2008/2009
=(2007*2009-2008²)/2008*2009
=[(2008-1)(2008+1)-2008²]/2008*2009
=(2008²-1-2008²)/2008*2009
=-1/2008*2009
a