原式=(x-y)/2(x+y)-(x²+y²)/(x+y)(x-y)=[(x-y)²-2(x²+y²)/[2(x+y)(x-y)]=(x²-2xy+y²-2x²-2y²)/[2(x+y)(x-y)]=(-x²-2xy-y²)/[2(x+y)(x-y)]=-(x+y)²/[2(...