how to write the polar form of $x^2/(x^2 + y^2)^2 - y^2/(x^2 + y^2)^2$? $$x^2/(x^2 + y^2)^2 - y^2/(x^2 + y^2)^2\\
r=(x^2+y^2)^{1/2}\\
x^2/r^4-y^2/r^4$$
Convert $x^2-y^2$ to polar form
$$x=r\cos(\theta)$$
$$y=r\sin(\theta)$$
$$(r\cos(\theta))^2-(r\sin(\theta))^2=r^2 \cos(2 θ)$$
$$r^2 \cos(2 θ)/r^4= \cos(2\theta)/r^2$$
Is that correct? 

Hint
It should look like this in the end:
$$E=x^2/(x^2 + y^2)^2 - y^2/(x^2 + y^2)^2=\frac {x^2- y^2}{(x^2 + y^2)^2}=\frac {\cos^2(\theta)- \sin^2(\theta)}{r^2}$$
$$E=\frac {2\cos^2(\theta)- 1}{r^2}=\frac {\cos(2\theta)}{r^2}$$
So it's correct...
