GATE CSE 2005 | Question: 40

Question

Let $P, Q,$ and $R$ be three atomic propositional assertions. Let $X$ denote $( P ∨ Q ) → R$ and $Y$ denote $(P → R) ∨ (Q → R).$ Which one of the following is a tautology?

• A. $X ≡ Y$
• B. $X → Y$
• C. $Y → X$
• D. $¬Y → X$

Comments

Option – D

counter example   (P, Q, R) = (True, True, False)

 

Answer 1

$X \equiv (P \vee Q) → R$
$\quad \equiv \neg(P \vee Q) \vee R$
$\quad \equiv (\neg P \wedge \neg Q) \vee R$
$\quad \equiv (\neg P \vee R) \wedge (\neg Q \vee R)$
$\quad \equiv (P →  R) \wedge (Q → R)$

So, $X  → Y$ is true as $(A \wedge B) → (A \vee B)$ is always TRUE but reverse implication is not always true.

Hence, B.

Comments

A→B is implication and B→ A is the reverse implication, in the same way, u can check that...

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why option A is wrong ?

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$X\equiv(P∨Q)→R$

$X\equiv\lnot(P∨Q)\vee R$

$X\equiv (\lnot P \wedge \lnot Q)\vee R$

$X\equiv \bar{P}.\bar{Q}+R$

$Y\equiv(P→R)∨(Q→R)$

$Y\equiv(\lnot P\vee R)∨(\lnot Q\vee R)$

$Y\equiv\bar{P}+R+\bar{Q}+R$

$Y\equiv\bar{P}+\bar{Q}+R$

How $X \equiv Y$ gives always true?

It is not possible to $X \equiv Y$ gives always true(Tautology).

Answer 2

A good explanation is already given by arjun sir. I'm going to share my approach- (using rules of boolean algebra, checking options one by one)

$X=(P\vee Q)\rightarrow r$

     $=\overline{p+q} +R$

     $=\bar{P}• \bar{Q} +R$

$Y=(P\rightarrow R) \vee (Q\rightarrow R)$

    $=(\bar{P} + R) + (\bar Q +R)$

    $=\bar P +\bar Q + R$

A. Clearly $X\;\text {not}\equiv Y$

B. X$\rightarrow$Y $=\bar X+Y$

     $=(\overline{\bar P \bar Q+R}) + (\bar P +\bar Q + R)$

     $=(P+Q) \color{green}{\bar R }\; +(\bar P +\bar Q)+ \color{green}R$

     $=(P+Q+R)(\bar R+R)+(\bar P+\bar Q)$

     $=(P+Q+R)+(\bar P+\bar Q)$

     $=1\;(TRUE)$  So option B is correct..

Comments

=(P+Q)R¯ + (P¯+Q¯) + R

 

=(P+Q+R)(R¯+R) + (P¯+Q¯)

 

Which rule is used to get this step ? Please, help me to understand.

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Distribution Law.

Answer 3

😊😊😊😊😊😊😊😊😊😊😊😊😊😊

Answer 4

Option B is correct.