2008²-2007×2009-999² =2008²-(2008-1)×(2008+1)-999² =2008²-(2008²-1²)-999² =1²-999²=(1+999)×(1-999)=1000×(-998)=-998000
2008²-2007×2009-999² =2008²-(2008^2-1)-999² =1-999^2=(1+999)(1-999)=1000x(-998)=-998000