z1-z2=500y(1+a+0.001)-500x(1+a)-500y(1+b+0.001)+500x(1+b)
=500[y+ya+0.001y-x-xa-y-yb-0.001y+x+xb]
=500[y(a-b)-x(a-b)]=500[(a-b)(y-x)]
y>x a>b
a-b>0 y-x>0 500[(a-b)(y-x)]>0
z1-z2>0 故z1>z2
z1-z2=500y(1+a+0.001)-500x(1+a)-500y(1+b+0.001)+500x(1+b)
=500[y+ya+0.001y-x-xa-y-yb-0.001y+x+xb]
=500[y(a-b)-x(a-b)]=500[(a-b)(y-x)]
y>x a>b
a-b>0 y-x>0 500[(a-b)(y-x)]>0
z1-z2>0 故z1>z2