Consider the following two well-formed formulas in prepositional logic.

F1: \(P \Longrightarrow \neg \;P\)

F2: \((P \Longrightarrow \neg \;P) \;\vee\; (\neg \; P \Longrightarrow P)\)

Which of the following statements is correct ? 

This question was previously asked in
UGC NET Computer Science (Paper 3) Nov 2017 Official Paper
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  1. F1 is Satisfiable, F2 is valid
  2. Fl is unsatisfiable, F2 is Satisfiable
  3. F1 is unsatisfiable, F2 is valid
  4. Fl and F2 both are Satisfiable

Answer (Detailed Solution Below)

Option 1 : F1 is Satisfiable, F2 is valid
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Detailed Solution

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The correct answer is option 1.

CONCEPT:

Valid: If for all the combinations of variables, the expression returns true then it is valid.

Satisfiable: If there exists at least one combination of variable which return true then it is satisfiable

Key Points

F1: It is Satisfiable.

P ¬P P⇒¬P
T F F
F T T
 

F2: It is valid

P ¬P P⇒¬P ¬PP (P⇒¬P)(¬PP)
T F F T T
F T T F T

∴ Hence the correct answer is F1 is Satisfiable, F2 is valid.

Additional Information

  • A proposition P is a tautology if it is true under all circumstances. It means it contains only TRUE in the final column of its truth table.
  • A statement that is always false is known as a contradiction.
  • A statement that can be either true or false depending on the truth values of its variables is called a contingency.
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