x²-y²-(x+y)=(x²-y²)-(x+y)=(x+y)(x-y)-(x+y)=(x+y)(x-y-1) 9-（5x-2y）²=[3+(5x-2y)][3-(5x-2y)]=(3+5x-2y)(3-5x+2y)
x2-y2-(x+y)=(x+y)(x-y)-(x+y)=(x+y)(x-y-1)9-(5x-2y)2=(3-5x+2y)(3+5x-2y)
