# 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.