4x²+4xy+y²-4x-2y+1=0
(2x+y)²-2(2x+y)+1=0
(2x+y-1)²=0
2x+y-1=0
2x+y=1
所以
2x²+3xy+y²-x-y
=(2x+y)(x+y)-x-y
=1*(x+y)-x-y
=x+y-x-y
=0