Answer to Question #103445 in Discrete Mathematics for samketi

Question #103445
Using the method of "searching for a falsifying truth assignment" prove the
De Morgan's law
1
Expert's answer
2020-02-24T09:44:00-0500

DeMorgan's Laws : 1. (AB)=AB(A\cap B)'=A'\cup B'

2. (AB)=AB(A\cup B)'=A'\cap B'

Let us assume (2) is false, and x(AB)    x(AB)x\in (A\cup B)' \implies x \notin (A\cup B)

    xAandxB    xAandxB\implies x\notin A and x\notin B \implies x\in A' and x\in B'

xABx\in A'\cap B' .

As x was a general element; (AB)=AB(A\cup B)'=A'\cap B' , which is a contradiction.

Hence, our assumption was false and (2) is true.

Similarly, (1) can be proved as well.



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