# Math Help - Analysis proof

1. ## Analysis proof

How would I prove there exists a positive real number x such that x^2=2? I need to use the Archimedean property.

2. Originally Posted by tharris
How would I prove there exists a positive real number x such that x^2=2? I need to use the Archimedean property.
Suppose there wasn't a root, appeal to the completeness of $\mathbb{R}$ and derive a contradiction using the Archimedean principle.

3. Do I start with: suppose x^2 does not equal 2? Or do I begin with assuming either a lower bound or an upper bound?

4. Originally Posted by tharris
Do I start with: suppose x^2 does not equal 2? Or do I begin with assuming either a lower bound or an upper bound?
Assume that $x^2\ne 2,\text{ }\forall x\in\mathbb{R}$ and think about the set $\left\{x\in\mathbb{R}:x^2>2\right\}$. By assumption it has a infimum, right? But what then?

5. this implies that the infimum is 0 which is a contradiction since x^2 >2?

6. Originally Posted by tharris
this implies that the infimum is 0 which is a contradiction since x^2 >2?
Well, the point is that it must have an infimum $\gamma$. But, $\gamma\in\mathbb{R}$ and so $\gamma^2\ne 0$ and so argue that $\gamma^2>2\implies \gamma^2-2>0$ but Archimedean implies what?

7. I guess I don't see the connection. The arcimedian principle states that if x>0, then there exists a positive integer n such that 1/n <x.
How does this relate to the infimum?

8. Originally Posted by tharris
I guess I don't see the connection. The arcimedian principle states that if x>0, then there exists a positive integer n such that 1/n <x.
How does this relate to the infimum?

The Archimedean principle says (geometrically) that

0--------|------------------

in the below we can always find some $n\in\mathbb{N}$ such that

0----- $\frac{1}{n}$-----|-------------

So, now we're claiming that $\gamma$ is the infimum of this set. But, $\gamma^2\ne 2$ and so if you argue correctly we have this scenario

0---- $\gamma^2-2$-----------

So, we should be able to "fit something" in between zero and $\gamma^2-2$ (by the A.P.)

but, what's the problem with that?

9. Does this imply that that "something" must be between 2 and "lambda" squared which is a contradiction since "lamda" squared is > 2?

10. Originally Posted by tharris
Does this imply that that "something" must be between 2 and "lambda" squared which is a contradiction since "lamda" squared is > 2?
A) $\gamma=\text{gamma},\lambda=\text{lambda}$

B) No. The point is that you can fit something squared in between $\gamma-2$ and zero and with a little algebra that something squared will be strictly greater than two but less than $\gamma$.

P.S. I don't know why I've been writing $\gamma^2$, it should always have been $\gamma$

11. Sorry, my greek symbols are rusty. You have been a great help. Does this stuff get more understandable? I hope so. So far I have that:
0 < 1/n < gamma - 2
If I add 2 to everything I get:
2 < 1/n + 2 < gamma

How did I show that 1/n + 2 is a square?

12. Originally Posted by tharris
Sorry, my greek symbols are rusty. You have been a great help. Does this stuff get more understandable? I hope so. So far I have that:
0 < 1/n < gamma - 2
If I add 2 to everything I get:
2 < 1/n + 2 < gamma

How did I show that 1/n + 2 is a square?
Of course it's a square $\frac{1}{n}+2=\sqrt{\frac{1}{n}+2}^2$