Sum of cosines with a multiplicative factor in the angle and different interval
5080
How can we sum up $\sin$ and $\cos$ series when the angles are in arithmetic progression?
24022
Compute $\int _0 ^{2\pi} (\sum_{n=1}^\infty \frac {\cos(nx)}{2^n})^2 dx$
155358
\sum caused \sqrt to not be slanted
116270
Functional equations leading to sine and cosine
90339
Integral involving Cosine function
83145
Sum of rational numbers
149850
Methods for determining the convergence of $\sum\frac{\cos n}{n}$ or $\sum\frac{\sin n}{n}$
114414
Sum of $\sum_{k=1}^n\sin{k\theta}$
7891
Sum of nth roots of unity
65698
Proving $\sum\limits_{k=0}^{n}\cos(kx)=\frac{1}{2}+\frac{\sin(\frac{2n+1}{2}x)}{2\sin(x/2)}$
14386
Proof of an equality involving cosine $\sqrt{2 + \sqrt{2 + \cdots + \sqrt{2 + \sqrt{2}}}}\ =\ 2\cos (\pi/2^{n+1})$
155367
summation of multiplying series with negative terms
146746
A formula for the sum of the triangular numbers?
108876
Sum of real numbers that multiply to 1
140984
Sum of Cosine/Exponential
94091
In $ \triangle ABC$ show that $ 1 \lt \cos A + \cos B + \cos C \le \frac 32$
108010
Evaluating $\sum_{k=1}^{\infty}\left(\frac{\sin(tk)}{k}\right)^2$