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Let A={1,2,3}A = \{1, 2, 3\}. Then, the number of relations containing (1,2)(1, 2) and (1,3)(1, 3) which are reflexive and symmetric but not transitive, is

Solution

✅ Correct Option: 1

We are given A={1,2,3}A = \{1, 2, 3\} and the relation must contain (1,2)(1, 2) and (1,3)(1, 3).


Since the relation is reflexive, (a,a)∈R(a, a) \in R for every a∈Aa \in A.

So RR must contain (1,1), (2,2), (3,3)(1,1),\ (2,2),\ (3,3).

Since the relation is symmetric, whenever (a,b)∈R(a, b) \in R, then (b,a)∈R(b, a) \in R.

(1,2)∈R⇒(2,1)∈R(1, 2) \in R \Rightarrow (2, 1) \in R

(1,3)∈R⇒(3,1)∈R(1, 3) \in R \Rightarrow (3, 1) \in R

So the minimum relation is:

Rmin⁡={(1,1), (2,2), (3,3), (1,2), (2,1), (1,3), (3,1)}R_{\min} = \{(1,1),\ (2,2),\ (3,3),\ (1,2),\ (2,1),\ (1,3),\ (3,1)\}


The only remaining symmetric pair that can optionally be included is (2,3)(2, 3) and (3,2)(3, 2).

This gives two candidate relations:

R1=Rmin⁡R_1 = R_{\min}

R2=Rmin⁡∪{(2,3), (3,2)}R_2 = R_{\min} \cup \{(2,3),\ (3,2)\}


Checking R1R_1 for transitivity:

(2,1)∈R1(2, 1) \in R_1 and (1,3)∈R1(1, 3) \in R_1, so we need (2,3)∈R1(2, 3) \in R_1.

But (2,3)∉R1(2, 3) \notin R_1, so transitivity fails.

R1R_1 is not transitive. ✓


Checking R2R_2 for transitivity:

R2R_2 contains all 9 pairs in A×AA \times A, making it the universal relation.

For any (a,b)(a, b) and (b,c)(b, c) in R2R_2, (a,c)(a, c) is always in R2R_2.

R2R_2 is transitive. ✗


Only R1R_1 satisfies all conditions: reflexive, symmetric, contains (1,2)(1,2) and (1,3)(1,3), but not transitive.

1\boxed{1}

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