x²-2x-4y²-4y
=(x²-4y²)-(2x+4y)
=(x+2y)(x-2y)-2(x+2y)
=(x+2y)(x-2y-2)
或
x²-2x-4y²-4y
=(x²-2x+1)-(4y²+4y+1)
=(x-1)²-(2y+1)²
=(x-1+2y+1)(x-1-2y-1)
=(x+2y)(x-2y-2)