1-（x-y）/(x+2y)÷(x²-y²)/(x+2y)²
=1-(x-y)/(x+2y)×(x+2y)²/(x+y)(x-y)
=1-(x+2y)/(x+y)
=(x+y-x-2y)/(x+y)
=-y/(x+y);
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