For the line $y = mx$, let $m = tan(\theta)$. Write $f(x, mx)$ as a function of $\theta$..? I have a problem, and I am not sure how to solve it.
This is the problem from my book:


  let $f(x, y)$ be given by the function: $$ f(x, y) =  \begin{cases} 
       \frac{2xy}{x^2 + y^2}, &amp; (x, y) \ne (0, 0) \\
       \
      0, &amp; (x, y) = (0, 0)      \end{cases} $$ for the line $y = mx$, let $m = tan(\theta)$. Write $f(x, mx)$ as a function of $\theta$. 
  
  Use the
  answer to the above to show how the value of $f(x, y)$ varies with the
  line’s angle of
  
  inclination.
Hint: Start by trying to write $f$ as a function of $r$ and $\theta$ (as in polar coordinates).
Hint: Start by trying to write $f$ as a function of $r$ and $\theta$ (as in polar coordinates).