1.x²-2x-3=（x-3)(x+1) 2.x²-2xy+y²-2x+2y+1 =(x-y)²-2(x-y)+1=(x-y-1）²
x²-2x-3=(x-3)(x+1)x²-2xy+y²-2x+2y+1=(x-y)²-2(x-y)+1=(x-y-1)²
