## kevo 3 years ago Mathematical Proofs

1. mr.luna

nice

2. kevo

Prove that $\exists z \in \mathbb{R} \forall x \in \mathbb{R}^+[\exists y \in \mathbb{R}(y-x+y/x) <--> x \neq z)$

3. kevo

Lol, sorry for the wait, it was a pain to type up.

4. Wheaton71

What is this

5. kevo

Proofs.

6. Wheaton71

Teach me

7. eliassaab

What does <−−> mean?

8. kevo

its the symbol for if and only if.

9. eliassaab

It is still not clear to me what the last statement mean. $\exists z \in \mathbb{R} \forall x \in \mathbb{R}^+[\exists y \in \mathbb{R}$ such that what?

10. Wheaton71

Yeah I'd also like to know.

11. kevo

What it says after R?

12. KingGeorge

just fyi, if you want to increase the space between characters in the equation editor, simply type "\;" for a small space, "\:" for a slightly bigger one, "\quad" for a big one, and "\qquad" for a giant one. This helps increase readability.

13. kevo

14. kevo

Thanks for the tip :)

15. eliassaab

Yes after R

16. KingGeorge

I think the best way to approach this would be to split it into two parts. First we want to show implication tot he right, and second we want to show implication to the right. Also note that $p \Rightarrow q \quad \Longleftrightarrow \neg \;p \;\;\text{V}\;\;q$

17. KingGeorge

So to show implication to the right, let's see if we can prove the simpler statement.

18. KingGeorge

I'm a little confused about the statement $$(y-x+y/x)$$. What is it saying? In this form it's virtually meaningless.

19. kevo

That's a good question.. so there exists y in R(y - x = y/x) iff x does not equal z.

20. kevo

This is just one confusing statement that should not be legal to give to student. Just saying.

21. KingGeorge

There's supposed to be an equals sign there. That helps. Give me a second to think about this.

22. kevo

LOL i just realized that I mistyped that. Sorry!

23. KingGeorge

Let's show implication to the right first. To show this, we need to choose a z such that for all y and x (x positive) $$(y-x =y/x)$$ or $$x\neq z$$. Just choose z to be negative. Since x is positive, we know that $$x \neq z$$.

24. KingGeorge

Now we need to show implication to the left. To show this, we need to choose a z such that for all positive x, there exists a y such that that $$z=x$$ or that $$y-x\neq y/x$$ Here, just choose $$y=0$$. Since x is positive, $$0-x$$ is less than 0, and $$0/x=0$$.

25. KingGeorge

Therefore, we are done. Sorry that took a while for me to write. Also, Instead of writing "we need to choose a z such that for all y and x (x positive)" in the first part, I should have written "we need to choose a z such that for all positive x there exists a y" It doesn't really matter in the end however.

26. kevo

KingGeorge. You are amazing. You are seriously my hero. I don't know how you are so good at this, but thank you.

27. KingGeorge

Practice, and a little bit of natural skill is how I'm good. I've also had some amazing teachers.

28. kevo

I know who I'm asking for help on proofs from now on :). I just don't know how I can reward you..

29. KingGeorge

As long as you're trying to learn, I'll be good.

30. kevo

Well, if you're ever in Seattle, I'll buy you dinner.

31. KingGeorge

Sounds good. :)