4x^2+y^2-4x-4y+5=0
4x^2-4x+1+y^2-4y+4=0
(2x-1)^2+(y-2)^2=0
x=1/2,y=2
x/y-y/x
=(1/2)/2-2/(1/2)
=1/4-4
=-15/4