1+2-3-4+5+6-7-8+9+10-11-12+...+2009
=(1-4)+(2-3)+(5-8)+(6-7)+(9-12)+(10-11)+...+2009
=-3+(-1)+(-3)+(-1)+...+2009
=-4+(-4)+(-4)+...(-4)+2009(502个-4,再加2009)
=1
1+2-3-4+5+6-7-8+9+10-11-12+...+2009
=(1-4)+(2-3)+(5-8)+(6-7)+(9-12)+(10-11)+...+2009
=-3+(-1)+(-3)+(-1)+...+2009
=-4+(-4)+(-4)+...(-4)+2009(502个-4,再加2009)
=1