13x^2-6xy+y^2-4x+1=0
(9x^2-6xy+y^2)+4x^2-4x+1=0
(3x-y)^2+(2x-1)^2=0
加数均非负,和为0.则均为0
所以有：
3x=y
2x-1=0
x=1/2
y=3/2
(xy-x^2)^3
=(3/4-1/4)^3
=1/8