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Prove validity of a cononclusion

  1. Mar 24, 2010 #1
    Prove the validity of the following:

    1. It rains, Ali is sick. Ali was not sick. ⊢ It didn't rain.

    2. I like maths, I study. I study or don't make an exam. ⊢ I don't make an exam, I do not like Maths.

    3. I study, I do not fail in maths. I don't play soccer, I study. I failed in maths. ⊢Therefore I played soccer.

    My attempts at solutions so far:

    1. ((p → q) Λ ¬q) → ¬p This statement is a tautology so this conclusion is true?

    2. Slightly trickier but this was my attempt. ((p → q) Λ (¬q V ¬r)) → (¬r → ¬p) This is not a tautology but has only one place that is false so is the argument true or not?

    3. ((p → ¬q) Λ (¬r → p) Λ q) → r This is also a tautology so this argument is valid?
     
  2. jcsd
  3. Mar 24, 2010 #2
    Just a question on number 2. If q is the proposition, I study, and r is the proposition, i make an exam. Then why is "I study or don't make an exam", [tex]\neg p \vee \neg r[/tex]?
     
  4. Mar 25, 2010 #3
    That was my mistake. Thanks for the correction. It should be [tex]\ p \vee \neg r[/tex]?

    So are my attempts at solutions correct? Since 1 and 3 are a tautology, they are right. The 2 is false, because it is not a tautology?
     
    Last edited: Mar 25, 2010
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