# Homework Help: Proving a sum that contains complex numbers

1. Oct 18, 2011

1. The problem statement, all variables and given/known data
show that:

I tried changing the form to the sin and cos, but I couldn't complete it..

Any hints?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

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2. Oct 18, 2011

### HallsofIvy

That's a geometric sequence. The sum of any geometric series is
$$\sum_{n=0}^\infty r^n= \frac{1}{1- r}$$

Here, your r is $e^{id}/2$.

3. Oct 18, 2011