\text{cos}(\varphi )+\text{cos}(\varphi +\alpha )+\text{cos}(\varphi +2\alpha )+\text{...}+\text{cos}(\varphi +n \alpha )=\frac{\text{sin}\left(\frac{(n+1)\alpha }{2}\right)\text{cos}\left(\varphi +\frac{n \alpha }{2}\right)}{\text{sin}\left(\frac{\alpha }{2}\right)}