2/1*3/1=6/1=2/1-3/1
3/1*4/1=12/1=3/1-4/1
4/1*5/1=20/1=4/1-5/1
.
2007/1*2008/1=2007/1-2008/1
(2/1*3/1)+(3/1*4/1)+(4/1*5/1)+.(2007/1*2008/1)
=2/1-3/1+3/1-4/1+4/1-5/1.+2007/1-2008/1
=2/1-2008/1
=2008/1004-1=2008/1003