Infinite sum of cosine function

1.3k Views Asked by At

What does the following expression equal to

$$\sum\limits_{n=1}^\infty \cos(n\cdot\theta)=\text{?}$$

1

There are 1 best solutions below

0
On

$$\sum\limits_{n=1}^m\cos(n\theta)=\Re\sum\limits_{n=1}^me^{in\theta}=\Re\frac{e^{i(m+1)\theta}-e^{i\theta}}{e^{i\theta}-1}.$$

This expression has no limit.