Prove if a function only have including the rational numbers and sin, cos, tan function and $r$, you always could write it in rectangular form. Ex. For $r=2/(2+2\cos\theta)$ it could be represent in rectangular form. Rectangular form is where the $r$ in the original function is $\sqrt{x^2+y^2}$ and you could create a function in terms of $x$ and $y$ that is $=0$ that is refer to the original function
Also, another question would be: What other trig function could involved in a function if it could be represent in rectangular form.
is there a complete answer for this question?