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

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In summary, to prove the statement (p implies q) or (q implies r), you need to show that it is a tautology, meaning it is true for all values of p, q, and r. This can be done by creating a truth table with 8 rows, one for each possible combination of truth values for p, q, and r.
  • #1
sombrancelha
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Homework Statement


I've started studying natural deduction recently and I have to prove

(p implies q) or (q implies r)

Homework Equations


--


The Attempt at a Solution


I thought about using the Law of Excluded Middle, but I'm not sure.
 
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  • #2
isn't that only one side of an expression - what do you have to prove?
 
  • #3
That's what I have to prove. There are no premisses.
 
  • #4
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?
 
  • #5
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.
 

1. What is the meaning of "p implies q"?

"p implies q" is a logical statement that means if p is true, then q must also be true. In other words, p is a sufficient condition for q to be true. This statement is also known as a conditional statement or an implication.

2. How is "p implies q" related to "q implies r"?

The statements "p implies q" and "q implies r" are related through a logical connective called "or". This means that at least one of the statements must be true for the overall statement to be true. In this case, either p being true implies q being true, or q being true implies r being true.

3. Can "p implies q" and "q implies r" both be false?

Yes, it is possible for both "p implies q" and "q implies r" to be false. This would happen if p is true but q is false, or if q is true but r is false. In this case, the overall statement would also be false because at least one of the individual statements is false.

4. How is "p implies q" different from "q implies p"?

"p implies q" and "q implies p" are not the same because they have different truth values. "p implies q" is true when p is true and q is true, whereas "q implies p" is true when q is true and p is true. The order of the statements matters in conditional statements.

5. What is the truth table for "p implies q" or "q implies r"?

The truth table for "p implies q" or "q implies r" would have four rows, representing all possible combinations of truth values for p, q, and r. The overall statement would be true if at least one of the individual statements is true, and it would be false only if both individual statements are false. The truth table would look like this:

p q r (p implies q) or (q implies r)
true true true true
true true false true
true false true true
true false false false

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