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

Give a proof or a counter-example for the following statements: (a) If $\phi$ is a contradiction, then $\phi$ is a tautology and $\psi$ is a contradiction. (b) if $\phi \land \psi$ is a tautology, then $\phi$ is a tautology or $\psi$ is a tautology. (c)