# Taylor Polynomial (can you help me?)

1. Mar 22, 2007

### kingwinner

1) Let f(x) = (x^3) [cos(x^2)].
a) Find P_(4n+3) (x) (the 4n + 3-rd Taylor polynomial of f(x) )
b) Find f^(n) (0) for all natural numbers n. (the n-th derivative of f evaluated at 0)

I know the definition of Taylor polynomial but I am still unable to do this quesiton. I tried to find the first few terms but I can't see an obvious pattern. I have no problem using the definition to find the first few terms...but this is a weird question. Can someone nicely give me some hints on both parts?

Any help is greatly apprecaited!

2. Mar 22, 2007

### Dick

Do you know a formula for the Taylor expansion of cos(x) around x=0? What does this imply for cos(x^2)?

3. Mar 22, 2007

### kingwinner

Yes, I know that formula, I guess I can replace x by x^2 and get a formula for cos(x^2) too...

4. Mar 22, 2007

### kingwinner

I also have another terrible quesiton...I am dying from it...

2) Find the 2006-th derivative evaluated at 0 and the 2007-th derivative evluated at 0 for f(x)=tan^(-1) x (inverse tan)

Any hints?

5. Mar 22, 2007

### Dick

I'm not very clear on what you mean f(x) to be.

6. Mar 22, 2007

### e(ho0n3

Find the first couple of derivates. Do you see a pattern? Can you determine a formula for the nth derivative of arctan from it? Can you prove your formula works for all n?

7. Mar 22, 2007

### kingwinner

There isn't an obvious pattern...what can I do?

8. Mar 22, 2007

### ZioX

Look up the generalized leibniz product rule.

9. Mar 22, 2007

### Dick

You could also simply look up the series expansion of arctan.