Let S be the set of all persons living in Delhi. We say that x, y in S are related if they were born in Delhi on the same day. Which one of the following is correct?

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NDA (Held On: 23 April 2017) Maths Previous Year paper
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  1. The relation is an equivalence relation
  2. The relation is not reflexive but it is symmetric and transitive
  3. The relation is not symmetric but it is reflexive and transitive
  4. The relation is not transitive but it is reflexive and symmetric

Answer (Detailed Solution Below)

Option 1 : The relation is an equivalence relation
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Detailed Solution

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Concept:

Definition of Equivalence relation

  • A relation is an equivalence relation if & only if it is:
  • Reflexive property: aRa
  • Symmetric property: if a R b, then b R a
  • Transitive property: if a R b and b R c, then a R c

 

Calculation:

Given that,

S = persons living in Delhi x, y ∈ S.

x R y if x & y born on same day.

⇒ First we check reflexive property so,

x R x or y R y both will be reflective.

So reflexive property true for Relation.

⇒ Now symmetric property,

Given that x R y born on same day so it’s reverse y R x also true y and x also born on same day.

Symmetric property also true for relation.

⇒ For transitive property,

Given that x R y born on same day

Let’s take z who also born on same day of y so y R z ∈ S

So we can say that x R z ∈ S because given that x R y and y R z ∈ S.

Transitive property also true for relation.

So this is an equivalence relation.

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