Hey Guys. I just wanted to see if someone can tell me how to denote "A or B but not both" in formal logic with BOTH SIDES of the contrapositive. Thanks in advance.
Edit: I meant to write If X > then A or B but not both. ( sorry).
Formal logc of " If X then A or B but not both"?
 MS415
 Posts: 48
 Joined: Mon Dec 03, 2012 3:02 pm
 jitsubruin
 Posts: 23
 Joined: Sun May 08, 2011 1:56 pm
Re: Formal logc of " If X then A or B but not both"?
A +B > not x
X > A or B
X > A or B
 dingbat
 Posts: 4976
 Joined: Wed Jan 11, 2012 9:12 pm
Re: Formal logc of " If X then A or B but not both"?
jitsubruin wrote:A +B > not x
X > A or B
not (A or B) > not X
 Cerebro
 Posts: 239
 Joined: Thu Aug 09, 2012 9:22 pm
Re: Formal logc of " If X then A or B but not both"?
dingbat wrote:jitsubruin wrote:A +B > not x
X > A or B
not (A or B) > not X
X  > [(A or B) & ~(A & B)]
 Cobretti
 Posts: 2562
 Joined: Tue Aug 21, 2012 12:45 am
Re: Formal logc of " If X then A or B but not both"?
Cerebro wrote:dingbat wrote:jitsubruin wrote:A +B > not x
X > A or B
not (A or B) > not X
X  > [(A or B) & ~(A & B)]
Dingbat's response = contrapositive to jitsu's second statement.
Cerebro's X > ~(A & B) = contrapositive to jitsu's first statement.
Everyone agrees!

 Posts: 3091
 Joined: Wed Mar 16, 2011 7:05 pm
Re: Formal logc of " If X then A or B but not both"?
X > (A or B) and (~A or ~B)
There are a few ways to write it (as seen above), but I find this to be the most straightforward, the easiest to remember, and the easiest to work with.
There are a few ways to write it (as seen above), but I find this to be the most straightforward, the easiest to remember, and the easiest to work with.

 Posts: 74
 Joined: Wed Jun 20, 2012 11:20 pm
Re: Formal logc of " If X then A or B but not both"?
Celebro is right, X > [ (A OR B) & ~(A & B) ] is the correct logical formulation.
I’m not sure I understood the suggestions for the contrapositive above. So I’ll just suggest my own.
X > [ (A OR B) & ~(A & B) ]
So reverse and negate BOTH SIDES .
~ [ (A OR B) and ~(A & B) ] > ~X
Then put the negation inside the brackets, and throw away the brackets.
~(A OR B) OR ~( A & B) > ~X
Then put the negations inside the parentheses.
(~A & ~B ) OR (~A OR ~B) > X
so (~A & ~B ) OR (~A OR ~B) > X is the contrapositive.
I’m not sure I understood the suggestions for the contrapositive above. So I’ll just suggest my own.
X > [ (A OR B) & ~(A & B) ]
So reverse and negate BOTH SIDES .
~ [ (A OR B) and ~(A & B) ] > ~X
Then put the negation inside the brackets, and throw away the brackets.
~(A OR B) OR ~( A & B) > ~X
Then put the negations inside the parentheses.
(~A & ~B ) OR (~A OR ~B) > X
so (~A & ~B ) OR (~A OR ~B) > X is the contrapositive.

 Posts: 3091
 Joined: Wed Mar 16, 2011 7:05 pm
Re: Formal logc of " If X then A or B but not both"?
TylerJonesMPLS wrote:so (~A & ~B ) OR (~A OR ~B) > X is the contrapositive.
I don't have time to see where you went wrong, but that's not the correct contrapositive. Especially since it's redundant  the first parenthetical is included in the second parenthetical. And you forgot the negation of X.
I'm going back to my formulation because there's no need to mess with moving negations around:
X > (A OR B) AND (~A OR ~B) (which is the same as Celebro, if you move the negation inside the second parenthetical instead of keeping it outside  it's more symmetrical that way)
(A OR B) is saying at least one of A or B; (~A OR ~B) is the same as saying not both A and B. Put them together and you get at least one, but not both.
Contrapositive:
(A AND B) OR (~A AND ~B) > ~X
If I have both, or if I have neither, I don't have X.

 Posts: 74
 Joined: Wed Jun 20, 2012 11:20 pm
Re: Formal logc of " If X then A or B but not both"?
The question is how to translate the sentence "If X then A or B but not both" into propositional logic, and to show the contrapositive.
Celebro's answer to the question of how to put the sentence in propositional logic is obviously correct: X > [(A OR B) AND ~(A AND B)]
(One can use DeMorgan's laws to derive (~A OR ~B) from ~(A AND B), but that requires a second step in the logic.)
I apologize for the typos and miscounting the negations in my earlier post; I wrote it out too quickly and carelessly. I will will begin with Celebro's formulation, and derive the contrapositive in two ways, because one may be easier to see than the other.
1. X > [(A or B) and ~(A and B)]
2. ~ [(A or B) and ~(A and B)] > ~X
3. [~(A or B) or ~~(A and B)] > ~X
4. [~(A or B) or (A and B)] > ~X
5. [(~A and ~B) or (A and B)] > ~X
1. X > [(A or B) and ~(A and B)]
2. X > [~(~A and ~B) and ~(A and B)]
3. ~[~(~A and ~B) and ~(A and B)] > ~X
4. [~~(~A and ~B) or ~~(A and B)] > ~X
5. (~A and ~B) or (A and B)] > ~X
Celebro's answer to the question of how to put the sentence in propositional logic is obviously correct: X > [(A OR B) AND ~(A AND B)]
(One can use DeMorgan's laws to derive (~A OR ~B) from ~(A AND B), but that requires a second step in the logic.)
I apologize for the typos and miscounting the negations in my earlier post; I wrote it out too quickly and carelessly. I will will begin with Celebro's formulation, and derive the contrapositive in two ways, because one may be easier to see than the other.
1. X > [(A or B) and ~(A and B)]
2. ~ [(A or B) and ~(A and B)] > ~X
3. [~(A or B) or ~~(A and B)] > ~X
4. [~(A or B) or (A and B)] > ~X
5. [(~A and ~B) or (A and B)] > ~X
1. X > [(A or B) and ~(A and B)]
2. X > [~(~A and ~B) and ~(A and B)]
3. ~[~(~A and ~B) and ~(A and B)] > ~X
4. [~~(~A and ~B) or ~~(A and B)] > ~X
5. (~A and ~B) or (A and B)] > ~X
Return to “LSAT Prep and Discussion Forum”
Who is online
Users browsing this forum: No registered users and 1 guest