Section 2.2: Problem 6 Solution
Working problems is a crucial part of learning mathematics. No one can learn... merely by poring over the definitions, theorems, and examples that are worked out in the text. One must work part of it out for oneself. To provide that opportunity is the purpose of the exercises.
James R. Munkres
Show that a formula is valid iff is valid.
is valid iff for every structure and , iff for every structure and , and every , (left to right, is a function from to , right to left, ) iff for every structure and , iff is valid.