（x-2y）²-（x+2y）²+2x²+8y²=[（x-2y）-（x+2y）]*[（x-2y）+（x+2y）]+2x²+8y²=[-4y]*[2x]+2x²+8y²= -8xy+2x²+8y²=2(-4xy+x²+4y²)=2(x-2y)²...