Question: let be a well formed formula whose only connective...
Let α be a well formed formula whose only connective symbols are ∧, ∨, and ¬. Let α∗ be the result of interchanging ∧ and ∨ and replacing each sentence symbol by its negation. Show that α∗ is tautologically equivalent to (¬ α). Use the induction principle.
Remark: It follows that if α |==| β then α∗ |==| β∗.