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Q.

Are the following conditional statements tautologies? 

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a

pqpq

b

pq⇒∼p

c

{(pq)p}qq

d

[(pq)p]qq

answer is A.

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Detailed Solution

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Truth table for the conditional statement pqpq

pqpqpqpqpq
TTTTT
TFFTT
FTFTT
FFFFT

Since all the truth values of the conditional 

pqpq are T, it is a tautology

(b) Truth table for the conditional statement

pq⇒∼p is as follows :

pqpq-Ppqp
TTTFF
TFFFT
FTFTT
FFFTT

Since all truth values of the conditional pq⇒∼p are neither T nor F so it is neither a tautology nor a contradiction. 

(c) Let us prepare the truth table for the conditional {(pq)p}qq

Let l={(pq)p}q Then we have

pqpq(pq)pl={(Pq)p}qlq
TTTTTT
TFFTFT
FTTTTT
FFTTFT

Thus the statement l=q i.e., {(pq)p}qq  has its truth value T for all its entries in the truth table. Hence, it is a tautology.

(d) We have to examine the statement

[(pq)p][q~q]

Let l=(pq)p and m=q~q

Then we have to examine the statement lm We now prepare the truth table for the conditional lm

pqpql=(pq)p--ql=(pq)pIm
TTTTFFF
TFFTTFF
FTFTFFF
FFFTTFF

Since all the truth values of the statement Imare not T, it is not a tautology. In fact, the statement

has its truth value F for all its entries in the truth table, it is a contradiction.Im

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