x^2+4y^2-4x+4y+5=0,求[（x^4-y^4)/xy]*[(2x^2-xy)/(xy-y^2)]/{(x^2+y^2)/y}^2.

x^2+4y^2-4x+4y+5=0
(x-2)^2+(2y+1)^2=0
所以有x-2=0 2y+1=0
所以x=2 y=-1/2
[（x^4-y^4)/xy]*[(2x^2-xy)/(xy-y^2)]/{(x^2+y^2)/y}^2.
=[(4-1/4)/1]*[(8-1)/(1-1/4)]/[(4+1/4)/1/4]^2
=35/289