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JEE Main 2021 · Mathematics · Sets, Relations and Functions · easy

The number of relations on the set A=\1,2,3,4\ that are both reflexive and symmetric is

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The number of relations on the set A={1,2,3,4}A=\{1,2,3,4\} that are both reflexive and symmetric is
  1. A. 6464
  2. B. 10241024
  3. C. 40964096
  4. D. 3232
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Answer: (A) 6464

A reflexive relation must contain all four pairs (a,a)(a,a). A symmetric relation is fixed by deciding, independently, whether each unordered pair {a,b}\{a,b\} with a≠ba\neq b is included. There are (42)=6\binom{4}{2}=6 such pairs, each included or not, so the count is 26=642^6=64. The values 10241024 (symmetric only, 2102^{10}) and 40964096 (reflexive only, 2122^{12}) count relations with just one of the two properties.
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