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Given p, we want to prove q. Which of the following will suffice:

(a) ¬q =⇒ ¬p

(b) p ∧ q =⇒ q

(c) ¬p ∧ ¬q =⇒ p

(d) ¬q =⇒ q

(e) p ∧ ¬q ∧ r =⇒ ¬r

(f) none of these

1 Answer

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Answers :

$(A)$ ~$Q →$ ~$P$

$(D)$ ~$ Q → Q$

The meaning of question as I understand :

We want to prove $Q$ when we know $P$ for sure. Meaning we want to say $P→ Q$.

Now question asks from which of the options we can infer $P→ Q$.

So question statement in short

Find $X$ such that $X→ (P→ Q)\equiv True$.

We know contrapositive of a statement is equivalent to that statement.

Means ~$Q →$ ~$P$ $\equiv$ $P→ Q$

$\therefore $ $($~$Q →$ ~$P) →  (P→ Q)$ is valid argument. So we don’t even need to check 1st option.


 

Solution : $P→ Q \equiv$ ~$P \vee Q$

${\color{Green} {Option\space1} }$ :  ~$Q →$ ~$P$  $\equiv Q$ $\vee$ ~$P$

Now, $ (Q$ $\vee$ ~$P)→$ $($~$P \vee Q)\equiv True$

${\color{Red} {Option\space2} }$ : $P \wedge Q → Q \equiv $ ~ $P$ $\vee$ ~$Q\vee Q\equiv True$

Now, $ True→ $ $($~$P \vee Q)\equiv\space ($~$P \vee Q) $ 

${\color{Red} {Option\space3} }$ : ~$P$ $\wedge$ ~$Q → P \equiv $  $P\vee Q\vee P\equiv P \vee Q$

Now, $ ( P \vee Q)→ $ $($~$P \vee Q)\equiv \space($~$P \vee Q) $

${\color{Green} {Option\space4} }$ :~$ Q → Q \equiv Q\vee Q\equiv Q$

Now, $ Q→$ $($~$P \vee Q) \equiv True$ 

${\color{Red} {Option\space5} }$ : $P$ $\wedge$ ~$ Q \wedge R→ $ ~$R \equiv $ ~$P$ $\vee$ $Q$ $\vee$ ~$R\vee R$ $\equiv True$ 

Now, $ True→$ $($~$P \vee Q)\equiv\space($~$P \vee Q)$

edited by

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