# Help to Prove f continuous

1. Mar 12, 2012

### IntroAnalysis

1. The problem statement, all variables and given/known data
Define f:[-1,∞]→ℝ as follows: f(0) = 1/2 and

f(x) =[(1 + x)^(1/2) - 1]/x , if x ≠ 0

Show that f is continuous at 0.

2. Relevant equations
Definition. f is continuous at xo if xoan element of domain and
lf(x) - f(xo)l < ε whenever lx - xol < δ

3. The attempt at a solution
Do some algebra come up with f(x) = 1/[(1+x)^(1/2) + 1]

I also know that between [-1,1], f(x)≤ 1
and x ≥ 1, f(x) ≤ 1

I know I need to somehow pull out an lxl from the absolute value, since I know lxl≤δ then
I can define δ in terms of ε and the function.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Mar 12, 2012

### SammyS

Staff Emeritus
Do you need to use an ε - δ proof, or can you use the limit criterion for continuity?
If $\displaystyle \lim_{x\to x_0}\,f(x)=f(x_0)\,,$ then f is continuous at x0 .​

3. Mar 12, 2012

### IntroAnalysis

It just says to show, it doesn't specify δ-ε proof. I've spent hours working on this one problem, any suggestions greatly appreciated!

4. Mar 12, 2012

### SammyS

Staff Emeritus
What is the following limit?
$\displaystyle \lim_{x\to0}\,\frac{1}{(1+x)^{1/2} + 1}$​

5. Mar 12, 2012

### IntroAnalysis

It is 1/2 which is f(0).
So this approach I show: 1) the point c is in the domain
2) the limit of f(c) exists and
3) lim x->c f(x)=f(c)

I should have thought of this, it is a lot easier to show. Thank you.

6. Mar 12, 2012

### SammyS

Staff Emeritus
You're welcome!