Y=\frac{4\frac{X}{4Y}}{4(\frac{X}{4Y})^2+1}= \frac{\frac{X}{Y}}{\frac{X^2}{4Y^2}+1} \cr =\frac{4XY^2}{Y(X^2+4Y^2)}= \frac{4XY}{X^2+4Y^2}