（4x-2y-1)^2+根号xy-2=0
4x-2y-1=0
xy-2=0
2x-y=1/2
xy=2
4x^2y-4x^2y^2-2xy^2
=2xy(2x-2xy-y)
=2*2(1/2-4)
=4*-7/2
=-14