999²-1002*998
=(1000-1)²-(1000+2)*(1000-2)
=1000²-2*1000*1+1²-(1000²-2²)
=1000²-2000+1-1000²+4
=-2000+1+4
=-1995
999²-1002*998=用简便方法计算
999²-1002*998
=(1000-1)²-(1000+2)*(1000-2)
=1000²-2*1000*1+1²-(1000²-2²)
=1000²-2000+1-1000²+4
=-2000+1+4
=-1995