x²-4x+4+|y-1|=0
(x-2)^2+|y-1|=0
x-2=0 x=2
y-1=0 y=1
（y/x - x/y）÷（x+y）
=(y^2-x^2)/(xy)*[1/(x+y)]
=(x+y)(y-x)/(xy)*[1/(x+y)]
=(y-x)/(xy)
=(1-2)/2
=-1/2