I would like to compute the tangential angle of a polynomial $f(x)$ as shown in this picture:
Here the red line represents $f(x)$.
According to this site of Wikipedia I can compute the tangential angle as follows
$\varphi = \arctan(f'(x))$
When I try to derive the equation for the tangential angle I end up with the following equation
$\varphi = \arctan(\frac{f(x)}{x})$
Which one is correct?