After trying to solve a geometry problem, represented by the following image
I've arrived at this expression:
$\alpha=\arccos\left(\frac{d+r \cos \left(\varphi+\frac{\vartheta}2\right)}{\sqrt{d^2+r^2+2 dr\cos \left(\varphi+\frac{\vartheta}2\right)}}\right)+\arccos\left(\frac{d+r\cos \left(\varphi-\frac{\vartheta}2\right)}{\sqrt{d^2+r^2+2dr\cos\left(\varphi-\frac{\vartheta}2\right)}}\right)$
Is there a way to simplify such expression?