1/3*5+1/5*7+1/7*9+...+1/1999*2001
=1/2(1/3-1/5+1/5-1/7+..+1/1999-1/2001)
=1/2(1/3-1/2001)
=1/2(2001-3)/6003
=999/6003
=111/667
1/3*5+1/5*7+1/7*9+...+1/1999*2001
=1/2(1/3-1/5+1/5-1/7+..+1/1999-1/2001)
=1/2(1/3-1/2001)
=1/2(2001-3)/6003
=999/6003
=111/667