- #1

rock.freak667

Homework Helper

- 6,230

- 31

**[SOLVED] Starting to prove by De Moivre's Theorem**

## Homework Statement

Use de Moivre's theorem to show that

[tex]\sum_{n=1} ^{\infty} \frac{sinn\theta}{2^n}=\frac{2^{N+1}sin\theta+sinN\theta-2sin(N+1)\theta}{2^N(5-4cos\theta)}[/tex]

## Homework Equations

[tex](cos\theta + isin\theta)^n=cosn\theta + isinn\theta[/tex]

## The Attempt at a Solution

Just need a little help on how to start this question. Where would I get the [itex]\frac{sinn\theta}{2^n}[/itex] from?