1-4(x-y)²=1-[2(x-y)]²=[1+2(x-y)][1-2(x-y)]=(1+2x-2y)(1-2x+2y) 4-x²-4xy-4y² =4-(x²+4xy+4y² )=4-(x+2y)²=(2+x+2y)(2-x-2y)