Choosing $\alpha$ and $\beta$ so that $f(x) = \frac{\sin (x + \alpha) \sin (x + \beta)}{\cos (x + \alpha)\cos (x + \beta)}$ is independent of $x$ 
Consider the function:
$$f(x) = \frac{\sin (x + \alpha) \sin (x + \beta)}{\cos (x + \alpha)\cos (x + \beta)}$$
Choose the parameters $\alpha$ and $\beta$ such that $f(x)$ does not
  depend on $x$.

Using Werner's formulas, I obtained:
$$f(x) = \frac{\cos(\alpha - \beta) - \cos(2x + \alpha + \beta)}{\cos(2x + \alpha + \beta) + \cos(\alpha - \beta)}$$
If $\alpha = \beta \pm \frac{\pi}{2}$, $f(x) = -1$.
Is this the only way to obtain a constant value from $f(x)$? I can't figure out if there are some alternatives, neither I am able to prove that there are not.

$f(x) = \frac{\sin(x + \alpha)\sin(x + \beta)}{\cos(x + \alpha)\cos(x + \beta)} = \frac{\cos(\alpha-\beta) - \cos(2x + \alpha + \beta)}{\cos(\alpha-\beta) + \cos(2x + \alpha + \beta)}$.
We need $f(x; \alpha, \beta) = const$, which is equivalent to $f'(x; \alpha, \beta) = 0, \forall x$.
$f'(x; \alpha, \beta) = \frac{4\sin(2x+\alpha + \beta)\cos(\alpha - \beta)}{\left(\cos(\alpha-\beta) + \cos(2x + \alpha + \beta)\right)^2} = 0, \forall x \Leftrightarrow \cos(\alpha-\beta) = 0 \Leftrightarrow \alpha-\beta = \frac{\pi}{2} + \pi n, n = 0, \pm 1, \pm 2, \ldots$.
