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Sets and Relations question

2024 · 30 Jan · Shift 2 · Q58
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  5. /2024 · 30 Jan · Shift 2 · Q58

Sets and Relations question

2024 · 30 Jan · Shift 2 · Q58

JEE MainMathematicsSets and RelationsNumerical+4 / −1
The number of symmetric relations defined on the set {1,2,3,4}\{1,2,3,4\}{1,2,3,4} which are not reflexive is ‾\underline{\hspace{2cm}}​.
Numerical answer
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Correct answer: 960

  1. Let the set be A={1,2,3,4}.A=\{1,2,3,4\}.A={1,2,3,4}. A relation on AAA is a subset of A×AA\times AA×A.

  2. For a relation to be symmetric, whenever (a,b)(a,b)(a,b) is in the relation, (b,a)(b,a)(b,a) must also be in the relation.

  3. Count all symmetric relations on AAA.

  • Diagonal pairs are: (1,1),(2,2),(3,3),(4,4).(1,1),(2,2),(3,3),(4,4).(1,1),(2,2),(3,3),(4,4). Each of these can be chosen independently, since each is automatically symmetric. So number of choices for diagonal pairs is 24.2^4.24.

  • Off-diagonal pairs come in unordered symmetric pairs: {(1,2),(2,1)},{(1,3),(3,1)},{(1,4),(4,1)},{(2,3),(3,2)},{(2,4),(4,2)},{(3,4),(4,3)}.\{(1,2),(2,1)\},\{(1,3),(3,1)\},\{(1,4),(4,1)\},\{(2,3),(3,2)\},\{(2,4),(4,2)\},\{(3,4),(4,3)\}.{(1,2),(2,1)},{(1,3),(3,1)},{(1,4),(4,1)},{(2,3),(3,2)},{(2,4),(4,2)},{(3,4),(4,3)}. There are (42)=6\binom{4}{2}=6(24​)=6 such pairs. For each such pair, either both are included or both are excluded. So number of choices for off-diagonal part is 26.2^6.26.

Hence total number of symmetric relations is 24⋅26=210=1024.2^4\cdot 2^6=2^{10}=1024.24⋅26=210=1024.

  1. Now subtract the symmetric relations that are reflexive.

A relation is reflexive if all diagonal pairs (1,1),(2,2),(3,3),(4,4)(1,1),(2,2),(3,3),(4,4)(1,1),(2,2),(3,3),(4,4) are present.

So for reflexive symmetric relations:

  • Diagonal entries are fixed: only 111 choice.
  • Off-diagonal symmetric pairs still have 262^626 choices.

Thus number of reflexive symmetric relations is 26=64.2^6=64.26=64.

  1. Therefore, the number of symmetric relations that are not reflexive is 1024−64=960.1024-64=960.1024−64=960.

So the required answer is 960.\boxed{960}.960​.

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