3-6x+3x=3-3x=3(1-x) x²+2x+1-y²=(x+1)²-y²=(x+1+y)(x+1-y)
x（x+y)-x-y=x(x+y)-(x+y)=(x-1)(x+y)
x²-y²-2x+1=x²-2x+1-y²=(x-1）²-y²=（x-1+y)(x-1-y)