Chapter 3
Review Exercises II
Informal Proofs
Matching. Drag sentences from the right to the correct
location in the proof box. Don't print until you've come to
the final page of this exercise.
First show that...
1. If two sentences are logically true in SL, then they are
logically equivalent in SL.

P
and Q are logically true sentences of SL.
by
definition of logically true in SL, both
P and Q are
true on every truth value assignment.
there
can be no truth value assignment making one of P
and Q true and the other false.
(None makes either false.)
by definition of equivalence, P
and Q are logically equivalent in SL.
Suppose
__________
Then,
__________
So,
__________
So,
finally, __________
Q.E.D.