(2x-y)(2x+y)-(2x-y)²=(2x-y)(2x+y-2x+y)=(2x-y)×2y=(2-2)×2×2=0
(2x-y)(2x+y)-(2x-y)²=4x²-y²-4x²+4xy-y²=4xy-2y²(其中x=1,y=2)=8-8=0