# De moivre's theorem complex number

1. May 31, 2014

### kelvin macks

can anyone explain how ro make the working above the red circle to the working in the red circle? why the author do this?

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2. May 31, 2014

### Mentallic

It's equivalent to having

(a+b)c = ac+bc

where

$$a=z^2+\frac{1}{z^2}$$

$$b=2$$

$$c=z^2-\frac{1}{z^2}$$

and why he did it should be pretty evident from his next two lines.

3. May 31, 2014

### HallsofIvy

Staff Emeritus
It has nothing to do with "complex numbers" or "DeMoivre's Theorem". It is, as mentallic said, just the distributive law.

4. May 31, 2014

### harmonic_lens

Hahaha, this is something Feynman talked about from his childhood. What you've presented is actually a higher-order form of something called Morrie's[/PLAIN] [Broken] Law (Feynman's little friend in childhood). From what I've studied, a useful application is in the proof of Urquhart's Theorem.

Last edited by a moderator: May 6, 2017