Proving this summation?

1. Jan 12, 2017

Vitani11

1. The problem statement, all variables and given/known data
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2. Relevant equations

3. The attempt at a solution

I have literally never seen anything like this in my life. I'm in mathematical physics. I looked up de Moivre's formula and I guess this comes from a course in complex variables? I don't know why this is a homework question given that nobody in my class has actually taken it, but whatever. I need to be able to prove this. Can you please help me out, where should I start? I am familiar with geometric series and I guess I can reference De Moivre's formula. That's where I'm at. My initial thought was that this had something to do with a taylor expansion, but I know cos(x)'s taylor expansion and I don't think that has anything to do with this.

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2. Jan 12, 2017

TeethWhitener

Write out de Moivre's formula and see if it has the same form as one of the summands you're given.

3. Jan 12, 2017

LCKurtz

Start by posting an image that is right side up.

4. Jan 12, 2017

Ray Vickson

Better still: do not post an image at all; type out the problem (the much preferred PF way).

5. Jan 14, 2017

Vitani11

Okay.

6. Jan 14, 2017

Vitani11

Got it, thank you Teeth