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Homework Help: Logic: (p implies q) or (q implies r)

  1. Aug 23, 2010 #1
    1. The problem statement, all variables and given/known data
    I've started studying natural deduction recently and I have to prove

    (p implies q) or (q implies r)

    2. Relevant equations

    3. The attempt at a solution
    I thought about using the Law of Excluded Middle, but I'm not sure.
  2. jcsd
  3. Aug 23, 2010 #2


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    isn't that only one side of an expression - what do you have to prove?
  4. Aug 26, 2010 #3
    That's what I have to prove. There are no premisses.
  5. Aug 26, 2010 #4


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    could just be me, but still not getting it... usuallly you have something like, "knowing A, prove B" but you just have B?

    do you mean you need to simplify or expand the expression?
  6. Aug 26, 2010 #5


    Staff: Mentor

    I believe that you are supposed to show that the given expression is a tautology. IOW, an expression that is true for all values of p, q, and r. A simple way to do this is to make a truth table with 8 rows, one for each of the possible truth values for p, q, and r.
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