原式=[(x+y)/(x+y)-x/(x+y)+2y²/(x+y)(x-y)]÷(x-y+2y)/(x-y)
=[y/(x+y)+2y²/(x+y)(x-y)]×(x-y)/(x+y)
=(xy-y²+2y²)/(x+y)(x-y)×(x-y)/(x+y)
=y(x+y)/(x+y)×1/(x+y)
=y/(x+y)