x-y分之2x的平方-y-x分之x的平方-4xy+x-y分之2y的平方-x的平方
=2x/(x-y)-(x²-4xy)/(y-x)+(2y-x²)/(x-y)
=(2x+x²-4xy+2y²-x²)/(x-y)
=(2x-4xy+2y²)/(x-y)