x的平方+2xy+y的平方分之x的平方-4y的评方 ÷ x的平方+xy分之x+2y
=(x+2y)(x-2y)/(x+y)²×x(x+y)/(x+2y)
=x(x-2y)/(x+y)
=(x²-2xy)/(x+y)