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Homework Help: Propositional Logic

  1. Sep 14, 2012 #1
    1. The problem statement, all variables and given/known data
    You cannot edit a protected Wikipedia entry unless you
    are an administrator. Express your answer in terms of e:
    “You can edit a protected Wikipedia entry” and a:“You
    are an administrator."

    I thought the answer would be [itex]a\rightarrow\neg e[/itex]; but the actual answer is,
    [itex]e\rightarrow a[/itex]. Why is that?
  2. jcsd
  3. Sep 15, 2012 #2


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    Science Advisor

    "¬e" says "you can NOT edit a protected Wikipedia entry" so "a→¬e" says "if you are an administrator you can NOT edit a protected Wikipedia entry"- not at all what you want to say.

    "You cannot edit a protected Wikipedia entry unless you are an administrator" is the same as "If you are not a Wikipedia administrator, you cannot edit a protected Wikipedia entry", ¬a→¬e and that is the same as e→a.
  4. Sep 15, 2012 #3
    All right, I have yet another one:
    You can graduate only if you have completed the requirements of your major and you do not owe money to the university and you do not have an overdue library book. Express your answer in terms of g: “You can graduate,” m: “You owe money to the university,” r: “You have com-pleted the requirements of your major,” and b:“You have an overdue library book.”

    I wrote my answer to be [itex](r \wedge \neg m \wedge \neg b)\rightarrow g[/itex]

    Yet, the answer key says the answer is the reverse. I don't really understand.
  5. Sep 18, 2012 #4
    Does anyone have any input?
  6. Sep 18, 2012 #5
    What do you mean the reverse?
  7. Sep 19, 2012 #6
    g → (r∧¬m∧¬b) like this. Sorry, I can see how that would be confusing.
  8. Sep 19, 2012 #7
    It may help to remember that "only if" is not the same as "if". P only if Q means P->Q but P if Q means Q->P. (I *think*)
  9. Sep 19, 2012 #8
    That one is tricky. The clue is "only if." a only if b translates as b->a. Your answer could be a true statement, but they weren't looking for a true statement, they were looking for a literal translation.
  10. Sep 19, 2012 #9
    Did you mean that a only if b translates to a->b?
  11. Sep 22, 2012 #10

    Whoops! You are right. That's what 17 years after grad school will do.
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