# Difference between revisions of "TU Wien:Einführung in wissensbasierte Systeme VU (Egly)/Übungen WS12/Blatt 1 - Beispiel 4"

(a) If $\phi$ is a contradiction, then $\phi$ is a tautology and $\psi$ is a contradiction.
(b) If $\phi \lor \psi$ is a tautology, then $\phi$ is a tautology or $\psi$ is a tautology.
(c) If $\phi$ and $\psi$ have no propositional letter in common, then $\phi \lor \psi$ is a tautology iff $\phi$ is a tautology or $\psi$ is a tautology.