Logic Proof Help

[She]

Good night, I´m trying t o learn by myself a little about formal logic, but it has been difficult. I understand what are the logic rules involved but I don´t know how to solve problems.

For example, I don't know how to solve this:



1. (F∨E)→~D
2. S∨D
3. E ∴ S

It seems easy, but I don't even know where to start. If someone could help me, I would be very grateful.

Thanks.

PS: If you could validate it with this checker, better yet it's proof #3: http://www.poweroflogic.com/cgi/Proof/checkprf.cgi?exercise=8.1C

[ydoaPs]

For any proposition, you can add anything at all to it with or. If you know p, then you also know p or q. See what you can do with that.

[md65536]

Where to start...

 

Do you know what you're trying to prove? ("Check proof now" gives a huge hint)

Can you explain in plain English informally how you'd prove it?

Does the "example proof" at your link make sense?

Do you know the name of the rule that ydoaPs is talking about?

Edited February 2, 2014 by md65536

[She]

@ydoaPs

 

I think you don't even know what you are talking about, "if P then q" is just one conditional. Solve the problem if you know how to. Thank you.

 

@md65536

 

The same here, solve the problem if you know how to, and check it. Do you know what should be proved? The conclusion is S, do you know how to reach it?

[Phi for All]

!

Moderator Note

The replies are attempting to "Help". We don't have a "Homework Answers" section.

 

Sorry.

[md65536]

Yes, must prove S. What do you know about E? What can you say about (FvE), or D, etc?

 

 

Since this is "homework help" I don't think I can just give a solution, but try this simpler example.

 

1. P -> Q

2. P :. Q

 

What are we proving? Given premises P -> Q, P, prove the conclusion Q.

How, informally? We're given that P is true. Since P implies Q, Q is also true.

How, formally? Modus ponens is the rule to use. Do you know what the proof should look like?

[She]

!

Moderator Note

The replies are attempting to "Help". We don't have a "Homework Answers" section.

 

Sorry.

Where am I saying it is homework? it is Not. I'm just asking if someone who really knows about this could help me. Until now, no such person has answered. Thank you for your "help".

Yes, must prove S. What do you know about E? What can you say about (FvE), or D, etc?

 

 

Since this is "homework help" I don't think I can just give a solution, but try this simpler example.

 

1. P -> Q

2. P :. Q

 

What are we proving? Given premises P -> Q, P, prove the conclusion Q.

How, informally? We're given that P is true. Since P implies Q, Q is also true.

How, formally? Modus ponens is the rule to use. Do you know what the proof should look like?

 

There are 3 premises:

 

1. (F∨E)→~D

2. S∨D

3. E

 

And a conclusion:

 

S

 

I know what a premise is, and what a conclusion is. Don't beat around the bush and help me solve it if you do know how to.

 

Thank You.

[Endy0816]

This site is more about making you think through a problem rather than giving you an answer. There are automated propositional logic solvers and other sites that might be better suited if all you desire is an answer.

 

My own process would be to first identify the commonality(D) and then look at my list of rules in an attempt to transform the equation as necessary. Something of a game of elimination.

 

If you could show what you've tried already or something of your thought process it would help a great deal in terms of getting you to a solution.

Edited February 3, 2014 by Endy0816

[John]

This proof is fairly straight-forward, and therefore it's difficult to really help without just giving the whole thing away. Whatever your intentions, the purpose of this forum is to give or receive hints not solutions.

 

Also, this website is giving me a headache.

Edited February 3, 2014 by ydoaPs
removed pic as it gave away too much of the answer

[ydoaPs]

Where am I saying it is homework? it is Not. I'm just asking if someone who really knows about this could help me. Until now, no such person has answered. Thank you for your "help".

 

 

There are 3 premises:

 

1. (F∨E)→~D

2. S∨D

3. E

 

And a conclusion:

 

S

 

I know what a premise is, and what a conclusion is. Don't beat around the bush and help me solve it if you do know how to.

 

Thank You.

I literally gave you the first step in the shortest (and most straightforward and obvious) proof.

 

You can use 'or' to add whatever you want to anything you already have. From there, it's two obvious steps.

[She]

Ok, I think no one knows. You don't have to "answer", or at least be honest and say you don't know. You are making others waste their time.

 

Thank you all for your "help".

Edited February 3, 2014 by She

[ydoaPs]

Ok, I think no one knows. You don't have to "answer", or at least be honest and say you don't know. You are making others waste their time.

 

Thank you all for your "help".

You've been given the rule for the next step multiple times. If you can't see it from that, you need to go back and review your rules as the last two are two of the most commonly used rules.

[Endy0816]

Ok, I think no one knows. You don't have to "answer", or at least be honest and say you don't know. You are making others waste their time.

 

Thank you all for your "help".

 

If (Nobody knows the answer), then (No answer presented)

(No answer presented)

Therefore (Nobody knows the answer)

 

1. If P, then Q.
2. Q.
3. Therefore, P.

Which is a logical fallacy. If nobody knows the answer then you can logically prove there will be no answer presented.

 

1. If P, then Q.
2. P.
3. Therefore, Q.

 

But there not being an answer presented does not prove that nobody knows the answer.

 

It benefits you to learn the process of thinking logically in a formal fashion via help, not through receiving an outright answer.

Edited February 4, 2014 by Endy0816

[She]

 

If (Nobody knows the answer), then (No answer presented)

(No answer presented)

Therefore (Nobody knows the answer)

 

1. If P, then Q.
2. Q.
3. Therefore, P.

Which is a logical fallacy. If nobody knows the answer then you can logically prove there will be no answer presented.

 

1. If P, then Q.
2. P.
3. Therefore, Q.

 

But there not being an answer presented does not prove that nobody knows the answer.

 

It benefits you to learn the process of thinking logically in a formal fashion via help, not through receiving an outright answer.

 

 

You've been given the rule for the next step multiple times. If you can't see it from that, you need to go back and review your rules as the last two are two of the most commonly used rules.

 

 

Just say you don't know. Be honest.

 

Good bye.

Edited February 4, 2014 by She

[hypervalent_iodine]

!

Moderator Note

Enough. This was posted in the homework help section, so it is reasonable to assume that you required help with the question. We have a policy here of not giving out answers to homework or even study questions, as doing so does not help the student asking the question(s) to learn anything.

The people posting here quite obviously do know the answer, but they are trying to help you answer it yourself rather than simply giving it to you. We aren't here to do your work for you, so please make an attempt at answering this on your own with or without the hints provided here, or explain where it is you are still struggling so that people may assist you better.

Being condescending or generally rude is not an appropriate response to people who are taking time out of their day to help you out. Please be more respectful in the future.

[She]

!

Moderator Note

The people posting here quite obviously do know the answer

 

I wouldn't be so sure of that, ha, ha, ha.

 

Thank goodness! these people will be very grateful with you Mr. Moderator, you have save them of making themselves look like fools. Is it so difficult to say: I don't know? They were not obliged to answer, were they?

 

 

If P then Q. Best answer to a proof by inference I have ever heard. It is like saying that a math problem is solved given 1 + 1 = 2. Brilliant!

 

They were not even able of giving me a hint about how would be the premise number 4. Reminder for me: "no to be so rude with people trying to "help".". And reminder for them should be: Not to talk about what they don't know.

 

Have a good night.

Edited February 4, 2014 by She

[Sensei]

I feel your frustration (especially because you don't seem to be apprentice, but private person).

But that's how Homework section works on this website.

You don't get instant answers for questions, rather hints that should direct you to the answer (and you have to do rest by yourself).

 

Do you saw Example Proof on that website?

http://www.poweroflogic.com/cgi/Proof/checkprf.cgi?usrchc=Example&exercise=8.1C

Click "Check Proof Now!" and you will see message "Congratulations: No errors were found in the proof.".

This is how proof on that website should looks like.

Edited February 4, 2014 by Sensei

[imatfaal]

!

Moderator Note

 

She

 

I will keep this simple - one more imputation of dishonesty directed at another member or the membership in general will lead to the staff considering a suspension of your posting rights.

 

This trolling attitude will not be tolerated. Not only is your contempt for the other members insulting and rude - it is massively misplaced; I am certain, through reading their posts for many years, that most of the members who have posted above understand your question very well indeed.

 

Do not respond to this moderation within the thread (they will be trash-canned) and do not continue with your present mode of posting.

 

 

 

Ok, I think no one knows. You don't have to "answer", or at least be honest and say you don't know. You are making others waste their time.

 

Thank you all for your "help".

 

 

 

 

 

 

Just say you don't know. Be honest.

 

Good bye.

 

 

 

I wouldn't be so sure of that, ha, ha, ha.

 

Thank goodness! these people will be very grateful with you Mr. Moderator, you have save them of making themselves look like fools. Is it so difficult to say: I don't know? They were not obliged to answer, were they?

 

 

If P then Q. Best answer to a proof by inference I have ever heard. It is like saying that a math problem is solved given 1 + 1 = 2. Brilliant!

 

They were not even able of giving me a hint about how would be the premise number 4. Reminder for me: "no to be so rude with people trying to "help".". And reminder for them should be: Not to talk about what they don't know.

 

Have a good night.

[Endy0816]

 

If P then Q. Best answer to a proof by inference I have ever heard.

 

You mean modus ponens? Keeps popping up. Weird how that happens, isn't it?

 

I only wish I could take credit for it. Greek thinkers and they really were quite brilliant.

 

If you are not trolling(which seems pretty likely at this point) I would guess that you are failing to understand the need for substitution and more broadly the relationship between statements you were told are true and equalities.

 

Really what you chose to do with this information is up to you at this point.

Edited February 5, 2014 by Endy0816

[She]

 

You mean modus ponens? Keeps popping up. Weird how that happens, isn't it?

 

I only wish I could take credit for it. Greek thinkers and they really were quite brilliant.

 

If you are not trolling(which seems pretty likely at this point) I would guess that you are failing to understand the need for substitution and more broadly the relationship between statements you were told are true and equalities.

 

Really what you chose to do with this information is up to you at this point.

 

Yet another "useful" answer...

 

Well, I guess this has been a completely waste of time. Thank you. It surprises me, you know what a basic Modus Ponens is! So 1 + 1 = 2 right? amazing.

 

I'm really starting to think, you really believe that you know what you are talking about. Self-delusion is called this.

That's it. I find that no one here knows about this, thank you very much for teaching me about nothing.

 

Reminder for all of you: if you don't know, you don't have to answer, it's ok to not know. What is shameful is to pretend you know about something of which you are clearly ignorant.

 

Regards.

Edited February 5, 2014 by She

[Endy0816]

 

Clearly.

 

Yeah, this is mostly for kicks and giggles at this point with OP suspended.

 

Seriously though, any members picking courses, I do recommend taking at least one that will teach you logic theory. Challenging, but more useful(real world) than others you'll take and promptly forget.

[imatfaal]

Which school is that Endy? - I guess from your mini-biog in the introduce yourself thread that this was as a "mature student". I did my law degrees in my 30s and it was a bit of surreal experience - being five years older by the calendar and five years younger by maturity than any other student on the course.

 

On the logic course - I might well take your advice and look out for one on edx.org or coursera.

 

Note to yDoaps - get Purdue to join edx.org and get a formal logic course together!