Let S denote all integers, define a relation R on S as aRb if ab ≥ 0 where a, b ∈ S’. Then R is :

  1. Reflexive but neither symmetric nor transitive relation
  2. Reflexive, symmetric but not transitive relation
  3. An equivalence relation
  4. Symmetric but neither reflexive nor transitive relation

Answer (Detailed Solution Below)

Option 2 : Reflexive, symmetric but not transitive relation
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NDA 01/2025: English Subject Test
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Detailed Solution

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Concept:

1. Reflexive: Each element is related to itself.

  • R is reflexive if for all x ∈ A, xRx.

2. Symmetric: If any one element is related to any other element, then the second element is related to the first.

  • R is symmetric if for all x, y ∈ A, if xRy, then yRx.

3. Transitive: If any one element is related to a second and that second element is related to a third, then the first element is related to the third.

  • R is transitive if for all x, y, z ∈ A, if xRy and yRz, then xRz.

4. R is an equivalence relation if A is nonempty and R is reflexive, symmetric and transitive.

Calculation:

S = Set of all integers and R = {(a, b), a, b ϵ  S and ab \(≥\) 0}

For reflexive:

aRa = a.a = a2 ≥ 0, so it's reflexive.

For symmetric:

aRb = ab ≥ 0 and bRa = ba ≥  0, So relation is symmetric

For transitive:

For all integers, if ab ≥ 0, bc ≥  0, then for all the cases ac ≥ 0 is not true.

For example,

If a = -1, b = 0, and c =1

Then,  ab ≥ 0, bc ≥  0 but for ac = -1 which is not satisfying ac ≥ 0 so R is not transitive.

So, Relation Is reflexive, symmetric but not transitive. 

Hence, option (2) is correct.

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