解：y-z=0 y=z x-y-z=0 x=y+z=2z xyz≠0 x≠0 y=z≠0原式=[1998（2z)^2+1999z^2-2000z^2]／[1998（2z）^2-1999z^2+2000z^2]=(1998*4-1)/(1998*4+1)=7991/7993
