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Homework Help: Why is this truth table incorrect?

  1. Sep 17, 2012 #1
    1. The problem statement, all variables and given/known data
    Create a truth table for ((P->Q) == (Q->P))

    2. Relevant equations

    3. The attempt at a solution


    I've had two quizes with problems just like this marked wrong in my college logic course. I confronted the professor about it and he just told me to study the truth tables. I'm very confused as to why this is wrong, because I've taken a discreet math course previously.
  2. jcsd
  3. Sep 17, 2012 #2


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    I don't think it is wrong. Looks perfectly fine to me.
  4. Sep 18, 2012 #3
    The conditionals for P and Q are both correct. Let's work with the third wff.

    Assume that (P → Q) ↔ (Q → P).

    Then, we have the same truth assignments as you just have! Indeed, like the other user just said, it's correct. Maybe, you should let your professor know about this! You are actually correct!
  5. Sep 18, 2012 #4
    Thank you for checking it. I went to talk to him again about it and he told me that the T/F values for P and Q were not in 'order', it's suppose to be:

    P | Q
    T | T
    T | F
    F | F
    F | T

    He was actually quite angry and scribbled on my paper incoherently with T's and F's. I've never been warned of this ordering in his class, and I guess that's what he means by memorizing the truth tables. Which seems kind of counterproductive =/
  6. Sep 18, 2012 #5


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    That ordering is completely arbitrary. I agree with you. It is counterproductive. Watch out for that guy.
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