# Thread: The principle of Taylor McLaurin

1. ## The principle of Taylor McLaurin

Please I need an explanation of what it is all about and how it is used to prove that: cosx plus sinx equals e^ix
Thanks

2. $\displaystyle \cos{x} + i\sin{x} = e^{ix}$

here's a good explanation ...

Fermat's Last Theorem: Euler's Formula

3. ## Maclaurin Series and Euler's Formula

Hi -

The general idea behind Maclaurin Series is to show you how to write functions like $\displaystyle sin x$, $\displaystyle cos x$ and $\displaystyle e^x$ as an infinite power series in $\displaystyle x$. (The Taylor series is similar, but is a bit more complicated. In fact, the Maclaurin is a special case of the Taylor series.)

If you don't know what I mean by an infinite power series, you're probably best to start by finding out about Infinite Geometric Series first. This will help you to understand how an infinite series can have a finite (and useful!) sum.

It's too complicated to show you here how the Maclaurin series is derived and used, but if you look at the .pdf file at http://mathinsite.bmth.ac.uk/pdf/macseries_theory.pdf you'll find a thorough explanation.

You need to look in particular at:

• Page 4, where you'll find the formula for the Maclaurin Series itself, and how it gives you the series for $\displaystyle sin x$ and $\displaystyle e^x$
• Page 6, where you'll find the series for $\displaystyle cos x$

To use these to prove Euler's formula, $\displaystyle cos x + i sin x = e^{ix}$ is very straightforward:

• Multiply both sides of the $\displaystyle sinx$ series by $\displaystyle i$, noting that $\displaystyle i^3 = -i, i^5=i$, etc.
• Add the result to the series for $\displaystyle cosx$, arranging the terms in ascending powers of $\displaystyle x$.
• In the series expansion of $\displaystyle e^x$, replace $\displaystyle x$ by $\displaystyle ix$. Note this time that $\displaystyle i^2=-1, i^4=1$, etc.
• Compare the result with what you found for $\displaystyle cosx+isinx$. You should find they're equal.

There's a proof in the Wikipedia article at Euler's formula - Wikipedia, the free encyclopedia.

Hope that helps.