# What did I do wrong here? (expressing root x as taylor series about a=4)

1. Mar 16, 2012

### skyturnred

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

Here is the question:

I don't quite know what I did wrong. My method is below.

2. Relevant equations

3. The attempt at a solution

f(x)=√x
f'(x)=$\frac{1}{2(x)^{1/2}}$
f''(x)=$\frac{-1}{(2)(2)(x^{3/2}}$

a=4
f(a)=2
f'(a)=1/4
f''(a)=$\frac{-1}{4(4^{3})^{1/2}}$

so wouldn't the second taylor polynomial be what I put in the answer field above? Thanks.

2. Mar 16, 2012

### SammyS

Staff Emeritus
What you have is the degree 2 term (almost). What's the sign?

Include the constant term & the linear term.

3. Mar 16, 2012

### skyturnred

Oh, so if it asks for T$_{2}$ it also wants the T$_{1}$ and the constant term as well?

Also, I see now that I somehow dropped a negative sign. Thanks!

EDIT: I just tried it and it's correct, so I answered my own question. But thanks again!