# Taylor series tips

#### toni

I really need some tips on taylor series...Im trying to learn it myself but i couldnt understand whats on the book...

Can anyone who has learnt this give me some tips...like whats the difference between it and power series (i know it's one kind of power series), why people develop it, and is there any standard way to prove that a function can be represent by a particular taylor series?

Thank you soooo much!

#### HallsofIvy

Homework Helper
A Taylor series is just a power series calculated in a particular way. Not only is it true that a Taylor series is a type of power series, but if a power series is equal to a function, it must be the Taylor series for that function.

That means I can calculate the Taylor series for, say, f(x)= 1/(1-x), at x= 0, in two different ways:
Using the definition, find the derivatives, evaluate at x= 0, and put those into the formlula: f(0)= 1, f'(x)= (1- x)=2 so f'(0)= 1, f"= 2(1-x)-3 so f"(0)= 2, ..., f(n)(x)= n!(1-x)n so fn[/sub](0)= n! and therefore,
$$\sum \frac{f^{(n)}(0)}{n!}x^n= \sum x^n$$

Or just recall that the sum of a geometric series, $\sum ar^n$ is 1/(1- r). Since 1/(1-x) this must be a geometric series with a= 1 and r= x: That gives
$$\frac{1}{1-x}= \sum x^n$$
just as before. Because they are power series converging to the same function, they musst be exactly the same.

#### mathman

The description given by Halls of Ivy is a special case of Taylor, call MacLauren (sp?) series. In general Taylor series involve powers of (x-a) where a is an arbitrary constant.

#### toni

is "a" arbitrary? "a" is also said to be the "center" right?

#### HallsofIvy

Homework Helper
If you mean the center of the interval of convergence, yes.

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