原式=x/(x-y)+y³/x(x-y)²÷[-y(x+y)/(x+y)(x-y)]=x/(x-y)-y³/x(x-y)²×(x-y)/y=x/(x-y)-y²/x(x-y)=(x²-y²)/x(x-y)=(x+y)(x-y)/x(x-y)=(x+y)/x