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A tautology is a Boolean formula that is always true. Which of the following is a tautology?

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A tautology is a Boolean formula that is always true. Which of the following is a tautology?
1. x
2. (x + x̅)y
3. x + y̅ + x̅
4. (xy) + x̅
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Correct Answer - Option 3 : x + y̅ + x̅

Calculation:

x + y̅ + x̅

= x + x̅ + y̅ 

= 1 + y̅        [∵ x + x̅ = 1]

= 1

So, it is always true.

Hence option 3 is correct.

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