GATE CSE 2014 | Set 3 | Question: 55

Question

Let $\oplus$ denote the exclusive OR (XOR) operation. Let '$1$' and '$0$' denote the binary constants. Consider the following Boolean expression for $F$ over two variables $P$ and $Q$:

$$F(P,Q)=\left( \left(1 \oplus P \right) \oplus \left( P \oplus Q \right )\right ) \oplus \left(\left(P \oplus Q\right) \oplus \left(Q \oplus 0\right)\right)$$

The equivalent expression for $F$ is

• A. $P+Q$
• B. $\overline{P+Q}$
• C. $P \oplus Q$
• D. $\overline {P \oplus Q}$

Comments

Detailed Video Solution: https://youtu.be/NF6DK5sS3S0?t=1616

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Option D

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Instead of solving and proving LHS = RHS we should try solving it by using values:

first try P=0 and Q=0 u will be left with option B and D

then try P=1 and Q = 1 there will be only D option left

Answer 1

XOR is associative and commutative. Also, $A \oplus A = 0$ and $A \oplus 1 = \overline{ A}$ and $A \oplus 0 = A$.  So
$\left( \left(1 \oplus P \right) \oplus \left( P \oplus Q \right )\right ) \oplus \left(\left(P \oplus Q\right) \oplus \left(Q \oplus 0\right)\right)$
$\implies \left(1 \oplus P \right) \oplus \left( \left( P \oplus Q \right ) \oplus \left(P \oplus Q \right) \right) \oplus \left(Q \oplus 0\right)$
$\implies  \left(1 \oplus 0 \right) \oplus \left( P \oplus Q \right) $
$\implies 1 \oplus \left(  P\oplus Q \right)$
$\implies \overline {\left( P \oplus Q\right)}$

Correct Answer: $D$

Comments

It is such a relief to not have to expand the options. -_-

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Sir but A (EXOR) 1 = A’ or A only ?

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@Arnav Singh_01 A'

Answer 2

D)
Since there are only 2 variables putting in pair of values of P and Q in F and checking with the options is a time saving method.
But Lets solve it.


 

Comments

Painful to see.

Efficient Approach: https://youtu.be/NF6DK5sS3S0?t=1616

Answer 3

Using the properties of associativity and commutativity, and the below mentioned properties we can find the correct answer:

1. $X⊕X = 0$

2. $X⊕1=X'$

3. $X⊕0=X$

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$F(P,Q)=((1⊕P)⊕(P⊕Q))⊕((P⊕Q)⊕(Q⊕0))$

$=(P'⊕(P⊕Q))⊕((P⊕Q)⊕(Q⊕0))$

$=((P'⊕P)⊕Q))⊕((P⊕Q)⊕(Q⊕0))$

$=((1⊕Q)⊕((P⊕Q)⊕(Q⊕0))$

$=(Q'⊕((P⊕Q)⊕(Q⊕0))) $

$=(Q'⊕((P⊕Q)⊕Q)) $

$=(Q'⊕(P⊕(Q⊕Q))) $

$=(Q'⊕(P⊕0)) $

$=(Q'⊕P) $

$=(Q⊕P)'$

So, the correct option is, option no. D.

Answer 4

Alternatively, You can follow this approach too:

Comments

exactly the way I did...

Answer 5

We need to simplify the above expression. As the given operation is XOR, we shall see property of XOR. Let A and B be boolean variable. In A XOR B, the result is 1 if both the bits/inputs are different, else 0. Now,

( ( 1 X P) X (P X Q) ) X ( (P X Q) X (Q X 0) )

( P' X P X Q ) X ( P X Q X Q ) ( as 1 X P = P' and Q X 0 = Q )

(1 X Q) X ( P X 0) ( as P' X P = 1 , and Q X Q = 0 )

Q' X P ( as 1 X Q = Q' and P X 0 = P )

PQ + P'Q' ( XOR Expansion, A X B = AB' + A'B )

This is the final simplified expression.

Now we need to check for the options.

If we simplify option D expression.

( P X Q )' = ( PQ' + P'Q )' ( XOR Expansion, A X B = AB' + A'B )

((PQ')'.(P'Q)') ( De Morgan's law )

( P'+ Q).(P + Q') ( De Morgan's law )

P'P + PQ + P'Q' + QQ'

PQ + P'Q' ( as PP' = 0 and QQ' = 0 ) 

Hence both the equations are same. Therefore Option D. 

Comments

you know the property from truth table of xor gate that is A(xor)0=A and A(xor)1=A'  A(xor)A=0 apply this rule on the above expression  

F(P,Q)=(1XOR P)XOR(PXORQ)XOR(PXORQXORQXOR 0)

=P'XOR(PXORQ)XOR(PXORQXORQ)

=P'XORPXORQXORP

=1XORQXORP

=Q'XORP

=(Q+P)'1

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Great explanation

Answer 6

observe the common term p ex or q in both

consider a case where p and q are equal

then p ex or q results in 0

1)in first p is ex ored  with 1

2)in second q is ex ored with 0

so if p, q are same then either of one oresults in 1 and another to 0

1 ex or  0

it is exnor