x-4x+4-y=(x-2)-=(x-2+y)(x-2-y)
原式=(x-2)²-y²=(x-2+y)(x-2-y)
x^2-4x+4-y^2=(x-2)^2-y^2=(x-2+y)(x-2-y)
=(x-2)-y=(x-2-y)(x-2+y)