原式=[(x-y)/(x+2y)]÷[(x²-y²)/(x²+4xy+4y²)]
=[(x-y)/(x+2y)]÷[(x+y(x-y)/(x+2y)²]
=[(x-y)/(x+2y)]×[(x+2y)²/(x+y)(x-y)]
=(x+2y)/(x+y)