# Calculating Taylor polynomials , Multiple Questions !

1. Jul 24, 2014

### TsVeryOwn

Whats up guys ! currently studying for calculas exam and could use someone going over my answers !

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

Q1. Calculate the taylor polynomial of degree 5 centred 0 for f(x) = e-x. Simply coeffcients and use the error formula to estimate the error when p5(0.1)

Q.2 Q1. Calculate the taylor polynomial of degree 6 centred 0 for f(x) = cos(x). Simply coeffcients and use the error formula to estimate the error when p6(-2)

Q3. Calculate the taylor polynomial of degree 3 centred x=1 for g(x) = x3/4. Simply coeffcients and use to approximate (0.9)3/4

2. Relevant equations

3. The attempt at a solution

Q1. 1 + x + -x2/2 - x3/6 + x4/24 + x5/120, , , 1.104 for next bit ?

Q2. 1- x2/2 + x4/24 + x6/720 ,, 2.422 for next part

Q3. Unsure on how to do

Could someone look over my answers , i think there right im just unsure about the polaritys of some of the figures and also im unsure if i answered it correctly when it asked me to "Estimate the errors"

P.S
Im new to this forum , im wondering if it would be possible to post up a PDF with around 8 diffrent questions on it ? i need them all answred for studying purposes as i have a repeat college exam coming up soon :uhh:
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jul 25, 2014

### Mogarrr

Below is an equation for a Taylor polynomial:

$P_n(x) = f(a) + f'(a)(x-a) + \frac {f''(a)}{2!}(x-a)^2 + ... + \frac {f^{(n)}(a)}{n!}(x-a)^{n}$.

The "error function" is also called the "remainder function", which is the following:

$R(x) = f(x) - P_{n,a}(x)$.

It's the difference between the value of the function at a certain point and the value of the Taylor polynomial centered at a.

Post if you have any troubles with the questions.

3. Jul 25, 2014

### Ray Vickson

That is NOT the error formula. The typical error formula would be
$$\text{error} = \frac{f^{(n+1)}(\xi)}{(n+1)!} (x-a)^{n+1},$$
where $\xi$ is a number in the interval $[a,x]$ (if $x > a$) or in $[x,a]$ (if $x < a$). See, eg., http://en.wikipedia.org/wiki/Taylor's_theorem .

Note: I mean that what you have done is define the error, not given a useful formula for it.

Last edited: Jul 25, 2014
4. Jul 25, 2014

### HallsofIvy

Staff Emeritus
You have your signs really confused here. Is "+ -x2" a typo? The Taylor polynomial for ex is 1+ x+ x2/2+ x3/6+ x4/24+ x5/120. To get the Taylor polynomial for e-x, replace x with -x which means every odd power will become negative.

If you want people to help you, you do the work of posting problems. Don't make the people you are asking for help do the work of opening and reading pdfs for you.

5. Jul 25, 2014

### TsVeryOwn

Cool thanks for the help guys but could yous maybe do out the question so i could see it more clearly ?

Q3. Calculate the taylor polynomial of degree 3 centred x=1 for g(x) = x3/4. Simply coeffcients and use to approximate (0.9)3/4

^^^This on in particular if someone does it step by step would be cool !

6. Jul 25, 2014

### Ray Vickson

It may be cool for you but it is very much against the Forum rules. You should read them; in particular, we are NOT allowed to do solutions step-by-step. We are allowed to help and give hints, etc., but YOU must do the work. If you ask more specific questions you might get better answers. Just saying you don't know how to do it is not good enough.