因为|xy-4|>=0,（x-2y-2）²>=0
所以由已知条件可得：、
xy-4=0
x-2y-2=0
故
xy=4
x-2y=2
从而可知
(-1/2xy)x²-4xy+4y²
=-2(x-2y)²
=-2*2²
=-8

（x+2y）²
=(x-2y)²+8xy
=4+8*4
=36