4x^2-4xy+y^2/2x^2-xy*4x^2-y^2/2xy+y^2
=(2x-y)²/x(2x-y)×(2x+y)(2x-y)/y(2x+y)
=(2x-y)/x×(2x-y)/y
=(2x-y)²/xy
,当x=1,y=2时
原式=(2-2)²/2×1=0