What is the definition of validity for a propositional logic formula?
PLz explain how to determine whether a given propositional logic formula with CNF is valid.

Please illustrate the different treatment for free and bound variables in the semantical evaluation of a predicate logic formula.

Please state the key idea of program verification using model checking techniques.

Prove the following sequent doesn't hold using semantical evaluation:
∀x∃y P(x,y) |- ∃y∀x P(x,y)