（4x-2y-1)^2+根号xy-2=0,
所以有：
4x-2y-1=0
xy-2=0
解得：2x-y=1/2 ,xy=2
2x^2y-4x^2y2-xy^2
=xy(2x-4xy-y)
=2(1/2-4*2)
=1-16
=-15