若丨x-y+3丨与丨x-y-1999丨互为相反数
丨x-y+3丨=0
丨x-y-1999丨=0
x-y+3=0
x-y-1999=0
x=998
y=1001
x+y/x-y=(1001+998)/(1001-998)=1999/3