x^2-2xy+y^2-2x+2y-3 =(x-y)^2-2(x-y)-3=(x-y-3)(x-y+1)
x^2-2xy+y^2-2x+2y-3=(x-y)^2-2(x-y)-3(x-y-3)(x-y+1)
原式=(x-y)²-2(x-y)-3=(x-y-3)(x-y+1)
令x-y=a 可得a^2 - 2a - 3 = (a+1)(a-3) 即 (x-y+1)(x-y-3)
