Geometric Series in Polar Form

Question

I'm stuck on a complex number problem relating to the geometric series in polar form. By considering that: $$1+\operatorname{cis}{\theta}+\operatorname{cis}{2\theta}+\operatorname{cis}{3\theta}+....+\operatorname{cis}{n\theta}$$ as a geometric series, find: $$\sum_{r=0}^n \operatorname{cos}{r\theta}$$ I was thinking along the lines of finding the sum of the polar form series, and that I was assuming that the sum of the series would be: $$\operatorname{cis}{\frac{n(n+1)}{2}}{\theta}$$ Thus, if that is the answer then finding the sum of $\operatorname{cos}{r\theta}$ becomes very easy. But I doubt that that is the answer, so any help would be much appreciated!!