2004x2003-2003x2002+2002x2001-2001x2000+...+2x1
= 2003(2004-2002)+2001(2000-1998)+...+3(4-2)+2*1
=2003*2+2001*2+...+3*2+1*2
=(2003+2001+...+3+1)*2
=(2003+1)^2/4*2
=2004^2/2
=2008008
2004*2003-2003*2002+2002*2001-2001*2000+...+2*1=2003(2004-2002)+...1(2-0)=2003*2+2001*2+...+1*2=2*1002*2005=4018020