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Without using truth table, show that ~r → ~ (p ∧ q) ≡ [~ (q → r)] → ~ p - Mathematics and Statistics

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Question

Without using truth table, show that

~r → ~ (p ∧ q) ≡ [~ (q → r)] → ~ p

Sum
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Solution

L.H.S.

≡ ~r → ~ (p ∧ q)

≡ ~(~ r) ∨ ~ (p ∧ q)       ....[p → q ≡ ~ p ∨ q]

≡ r ∨ ~(p ∧ q)             ....[Negation of negation]

≡ r ∨ (~p ∨ ~q)           ....[De Morgan’s law]

≡ ~p ∨ (~q ∨ r)         .....[Commutative and associative law]

≡ ~p ∨ (q → r)          ....[p → q ≡ ~ p ∨ q]

≡ (q → r) ∨ ~p            ......[Commutative law]

≡ ~[~ (q → r)] ∨ ~ p     ......[Negation of negation]

≡ [~ (q → r)] → ~ p        .....[p → q ≡ ~ p ∨ q]

= R.H.S.

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Chapter 1: Mathematical Logic - Exercise 1.9 [Page 22]

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