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anonymous
 one year ago
Provide a proof using standard predicate logic (without using the deduction method, proof by contradiction, or proof by contraposition) for the validity of the following argument:
(∀x)A(x) ∧ (∃x)B(x) → (∃x)[A(x) ∧ B(x)]
anonymous
 one year ago
Provide a proof using standard predicate logic (without using the deduction method, proof by contradiction, or proof by contraposition) for the validity of the following argument: (∀x)A(x) ∧ (∃x)B(x) → (∃x)[A(x) ∧ B(x)]

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anonymous
 one year ago
Best ResponseYou've already chosen the best response.0I'm trying to solve this now. If anyone wants to work with me then that would be great; otherwise, I have no way to check my proof.

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0If you care to show your solution I am happy to read it for you. What I remember from my course (which was very long ago) was to in order to show such statements we had to use axioms, in fact we had 13. It is indeed hard to judge if you're working with the same axiom system as I did. If you work with an axiom system you need to share your axioms in order to get help. Aside from that, I can only tell that the implication is indeed true. Because if a Formula A(x) is true for all its arguments and there exists an x such that B(x) is true then of course there exists an x such that A(x) and B(x) are true. This is however far from a formal proof. If you need a formal one, we need your axioms.

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0@Spacelimbus Thank you for replying. I like your informal proof :) I am still working on trying to figure this out.

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0@zepdrix how are you with predicate logic?

zepdrix
 one year ago
Best ResponseYou've already chosen the best response.0Ok sorry sorry was finishing up a game :d What's going on now?

zepdrix
 one year ago
Best ResponseYou've already chosen the best response.0If `For all x, A(x)` `and` `there exists an x such that B(x)`, then `there exists an x such that` `A(x) and B(x)`. This is really weird stuff 0_o I don't remember ever doing this nonsense with these quantifiers.. hmm

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0The good news is I'm fine with induction and propositional logic now... this is the last piece of the puzzle.

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0what game were you playing?

zepdrix
 one year ago
Best ResponseYou've already chosen the best response.0Had to give up the addicting games so I could really really focus on schooling this semester :P Rocket League is mindless fun though, nothing that I can't let go of.

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0that's cray. i've never seen a vehicle based soccer game before, lol

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0i only play chess and starcraft :X

zepdrix
 one year ago
Best ResponseYou've already chosen the best response.0Hmm I gotta think about this problem a bit :O

zepdrix
 one year ago
Best ResponseYou've already chosen the best response.0Ummm ok let's see if this works or not...

zepdrix
 one year ago
Best ResponseYou've already chosen the best response.0The first piece: \(\large\rm \forall x, A(x)\) For all x, A(x). So any x's that we have, all of them, satify this statement A. So all together we have the union of a bunch of statements A involving different x's. \[\large\rm \forall x, A(x)\quad\equiv\quad A(x_1)\cup A(x_2)\cup...\cup A(x_n)\]

zepdrix
 one year ago
Best ResponseYou've already chosen the best response.0This next piece: \(\large\rm \exists x: B(x)\) Which translates to `There exists an x such that B(x)`, tells us that there is a single x in existence that satisfies the statement B. Let's call it x_i, just one of the many x's we could list. \[\large\rm \exists x: B(x)\quad\equiv\quad B(x_i)\]

zepdrix
 one year ago
Best ResponseYou've already chosen the best response.0And of course, the caret shape is to denote "and" or "intersection.

zepdrix
 one year ago
Best ResponseYou've already chosen the best response.0So on the left side of our thing we have,\[\large\rm \left[A(x_1)\cup A(x_2)\cup...\cup A(x_n)\right]\cap B(x_i)\]

zepdrix
 one year ago
Best ResponseYou've already chosen the best response.0They only intersect at the same x,\[\large\rm A(x_i)\cap B(x_i)\]And we need to see if that `implies` the right part. I don't think I'm doing this right _ it's been too long...

zepdrix
 one year ago
Best ResponseYou've already chosen the best response.0On the right side, \(\large\rm \exists x:\left[A(x)\wedge B(x)\right]\) Translates to: There exists an x such that, both A(x) and B(x). Again, let's call this x, x_i. So the right side says that some x exists, so that the intersection of A(x) and B(x) is satisfied.\[\large\rm \exists x:\left[A(x)\wedge B(x)\right]\quad\equiv\quad A(x_i)\cap B(x_i)\]

zepdrix
 one year ago
Best ResponseYou've already chosen the best response.0But again, I don't think I'm doing this correctly _ There are probably some familiar easy steps that I'm forgetting about. I just don't remember how to deal with these XD lol
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