In: Advanced Math
Let p and q be propositions.
(i) Show (p →q) ≡ (p ∧ ¬q) →F
(ii.) Why does this equivalency allow us to use the proof by contradiction technique?
(i) p and q are two propositions.
Let us prove that p q and ( p
¬q F ) are
equivalent using a Truth Table.
p q ¬q p ¬q F p ¬q
F p
q
0 0 1 0 0 1 1
0 1 0 0 0 1 1
1 0 1 1 0 0 0
1 1 0 0 0 1 1
Since the corresponding truth values of p q and p
¬q F are the
same, hence (p q) ≡ (p
¬q) F
(ii) The technique of Proof by Contradiction is an Indirect Proof
technique. To prove an implication P Q,
we assume (on the contrary) that P and NOT Q are both true. Under
this antecedent, we prove that the Consequent is a Contradiction F
(that is, a statement F which is always false, in some given
logical system). Since a Contradiction arises, this therefore
implies that P Q is indeed
true. Hence, the equivalency P Q
( P
¬Q F ) allows
us to use the proof by contradiction technique.