Don't Booley me

Logic Level 2

( ( b ¬ c ) [ ( ¬ a b ) a ( ¬ b c ) ] ) ¬ a ( a c ) \left( (b \lor \neg c) \land \left[ (\neg a \land b) \lor a \lor (\neg b \land c) \right] \right) \lor \neg a \lor (a \land c)

Is the above statement true for all propositions a , b , c a,b,c ?

True False Depends

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3 solutions

Jake Lai
Jun 20, 2015

Let's convert this statement into Boolean algbera:

( b + c ) ( a b + a + b c ) + a + a c (b+\color{#D61F06}{\overline{c}})(\overline{a}b+\color{#D61F06}{a}+\overline{b}c)+\overline{a}+ac

Using the distributive law we see that there will be a a c \color{#D61F06}{a\overline{c}} term in the expansion. Since

a c + a + a c = a ( c + c ) + a = a + a = 1 a\overline{c}+\overline{a}+ac = a(\overline{c}+c)+\overline{a} = a+\overline{a} = 1

and p + 1 = 1 p+1 = 1 for any proposition p p , we get that the statement given is a tautology!

never seen half of these symbols...

Trevor Arashiro - 5 years, 11 months ago

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Learn something new everyday!

Calvin Lin Staff - 5 years, 11 months ago

Excellent solution.But I did it with truth tables .Didn't think of this method..

Krishna Shankar - 5 years, 11 months ago

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Drawing a truth table will be quite hectic for this.

Venkata Karthik Bandaru - 5 years, 11 months ago

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True but this method didn't click at that instant

Krishna Shankar - 5 years, 11 months ago
Shubham Jalan
Jun 30, 2015

Let there be a falsifying assignment. Then a must be assigned as 1 otherwise

                    ... +~a+ ac

will be true

Now for a.c to be false, c must be 0.

So, in (b+~c), ~c is true. In (~a.b + a + ~b.c), a is true. Hence (b+~c).(~a.b + a + ~b.c) is true.

So there can be no falsifying assignment. Hence the proposition is a tautology.

Arun Damodaran
Jun 30, 2015

There is no need to solve the entire expression.Here are few steps for easy evaluation

  1. Consider,

(b V ~c) ^(and) with remaining ((~a^b)VaV(~b^c))

whatever may be the RIGHT PART of '^' just neglect and MAKE (b V~c) as ZERO(FALSE) i.e b=0,c=1.

    Thus   when (and)ed with right part gives zero.
  1. Consider,

    (derived from previous step):- (FALSE)0 V ~a V(a^c)

already c=1, choose whatever values for 'a' it will yield TRUE always .

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