(x+y-4)(x-y-4)=[(x-4)+y][(x-4)-y]=(x-4)²-y²=x²-8x+16-y²
(x+y-4)(x-y-4)=x²-(y-4)²=x²-y²+8y-16
=[(x-4)+y][(x-4)-y]=(x-4)²-y²=x²-8x-y² +16
