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Sets and Relations question

2025 · 22 Jan · Shift 2 · Q50
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Sets and Relations question

2025 · 22 Jan · Shift 2 · Q50

JEE MainMathematicsSets and RelationsNumerical+4 / −1
Let A={1,2,3}A=\{1,2,3\}A={1,2,3}. The number of relations on AAA, containing (1,2)(1,2)(1,2) and (2,3)(2,3)(2,3), which are reflexive and transitive but not symmetric, is ‾\underline{\hspace{2cm}}​.
Numerical answer
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Correct answer: 3

  1. Set up all required pairs

For A={1,2,3}A=\{1,2,3\}A={1,2,3}, a relation RRR on AAA is a subset of A×AA\times AA×A.

Since RRR is reflexive, it must contain:

(1,1),(2,2),(3,3)(1,1),(2,2),(3,3)(1,1),(2,2),(3,3)

It is also given that RRR contains:

(1,2),(2,3)(1,2),(2,3)(1,2),(2,3)

So currently,

{(1,1),(2,2),(3,3),(1,2),(2,3)}⊆R\{(1,1),(2,2),(3,3),(1,2),(2,3)\}\subseteq R{(1,1),(2,2),(3,3),(1,2),(2,3)}⊆R
  1. Use transitivity

Since (1,2)∈R(1,2)\in R(1,2)∈R and (2,3)∈R(2,3)\in R(2,3)∈R, transitivity forces:

(1,3)∈R(1,3)\in R(1,3)∈R

Thus every such relation must contain at least:

(1,1),(2,2),(3,3),(1,2),(2,3),(1,3)(1,1),(2,2),(3,3),(1,2),(2,3),(1,3)(1,1),(2,2),(3,3),(1,2),(2,3),(1,3)
  1. List the only remaining optional pairs

The full set A×AA\times AA×A has 999 pairs. We already know 666 of them must be present. The remaining pairs are:

(2,1),(3,1),(3,2)(2,1),(3,1),(3,2)(2,1),(3,1),(3,2)

Let us decide which of these can be added while keeping transitivity, and then exclude symmetric relations.

  1. Check all subsets of {(2,1),(3,1),(3,2)}\{(2,1),(3,1),(3,2)\}{(2,1),(3,1),(3,2)}

We examine all 23=82^3=823=8 possibilities.


Case 1: Add none

Relation contains only

(1,1),(2,2),(3,3),(1,2),(2,3),(1,3)(1,1),(2,2),(3,3),(1,2),(2,3),(1,3)(1,1),(2,2),(3,3),(1,2),(2,3),(1,3)

This is reflexive and transitive. It is not symmetric since (1,2)∈R(1,2)\in R(1,2)∈R but (2,1)∉R(2,1)\notin R(2,1)∈/R.

So this case is valid.


Case 2: Add (2,1)(2,1)(2,1) only

Now (2,1)(2,1)(2,1) and (1,2)(1,2)(1,2) imply by transitivity nothing new beyond reflexive pairs, but:

  • (2,1)(2,1)(2,1) and (1,3)(1,3)(1,3) force (2,3)(2,3)(2,3), already present.
  • (1,2)(1,2)(1,2) and (2,1)(2,1)(2,1) force (1,1)(1,1)(1,1), already present.

So transitivity is preserved. It is still not symmetric because (2,3)∈R(2,3)\in R(2,3)∈R but (3,2)∉R(3,2)\notin R(3,2)∈/R.

Valid.


Case 3: Add (3,1)(3,1)(3,1) only

Then (2,3)(2,3)(2,3) and (3,1)(3,1)(3,1) force:

(2,1)(2,1)(2,1)

which is not present. So transitivity fails.

Invalid.


Case 4: Add (3,2)(3,2)(3,2) only

Then (1,2)(1,2)(1,2) and (2,3)(2,3)(2,3) already gave (1,3)(1,3)(1,3). Also (1,3)(1,3)(1,3) and (3,2)(3,2)(3,2) force:

(1,2)(1,2)(1,2)

which is already present. And (2,3)(2,3)(2,3) and (3,2)(3,2)(3,2) force (2,2)(2,2)(2,2), already present.

So transitivity holds. But check symmetry:

  • (2,3)(2,3)(2,3) and (3,2)(3,2)(3,2) are symmetric,
  • yet (1,2)∈R(1,2)\in R(1,2)∈R but (2,1)∉R(2,1)\notin R(2,1)∈/R. So relation is not symmetric.

Valid.


Case 5: Add (2,1),(3,1)(2,1),(3,1)(2,1),(3,1)

Since (2,3)(2,3)(2,3) and (3,1)(3,1)(3,1) imply (2,1)(2,1)(2,1), already present. Since (3,1)(3,1)(3,1) and (1,2)(1,2)(1,2) imply:

(3,2)(3,2)(3,2)

which is missing. So transitivity fails.

Invalid.


Case 6: Add (2,1),(3,2)(2,1),(3,2)(2,1),(3,2)

Check transitivity:

  • (2,1)(2,1)(2,1) and (1,3)(1,3)(1,3) imply (2,3)(2,3)(2,3), already present.
  • (3,2)(3,2)(3,2) and (2,1)(2,1)(2,1) imply (3,1)(3,1)(3,1), which is missing. So transitivity fails.

Invalid.


Case 7: Add (3,1),(3,2)(3,1),(3,2)(3,1),(3,2)

Since (2,3)(2,3)(2,3) and (3,1)(3,1)(3,1) imply (2,1)(2,1)(2,1), which is missing. So transitivity fails.

Invalid.


Case 8: Add all three: (2,1),(3,1),(3,2)(2,1),(3,1),(3,2)(2,1),(3,1),(3,2)

Then relation becomes all 999 pairs of A×AA\times AA×A. This is reflexive, transitive, and symmetric. But we need not symmetric.

Invalid.

  1. Count valid relations

Valid cases are:

  1. none added
  2. only (2,1)(2,1)(2,1)
  3. only (3,2)(3,2)(3,2)

Hence the number of required relations is:

333
  1. Compare with stored answer

Derived answer = 333. Stored correct answer = 333. They agree.

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