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Class 12MathematicsRelations and FunctionsEquivalence relationExpert
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On (A={1,2,3,4}), (aRb) if and only if (a+b) is prime. Why is this not an equivalence relation?

Answer and explanation

Correct answer: Because it is not reflexive

Step 1: Reflexivity would require (a+a) to be prime for every (a). Step 2: For (a=2), (2+2=4), which is not prime. Step 3: Therefore the relation is not reflexive on all elements and cannot be equivalence.

Related tags

Prime SumReflexivity FailureFinite SetRelationsEquivalence RelationRelations And FunctionsMathematicsClass 12 Mcq
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10 more MCQs
  1. 1What is the smallest change needed to make R={(1,1),(2,2),(3,3),(1,2),(2,1),(2,3),(3,2)} an equivalence relation on A={1,2,3}?
  2. 2For the relation on A={1,2,3,4} whose equivalence classes are {1,2} and {3,4}, which pair is definitely absent?
  3. 3How many ordered pairs does the equivalence relation with classes {1,2} and {3,4} contain?
  4. 4Which pair must belong to the smallest equivalence relation on A={1,2,3,4} containing R={(1,2),(2,3),(4,4)}?
  5. 5Which pair is not present in the smallest equivalence relation on A={1,2,3,4} containing R={(1,2),(2,3),(3,1)}?
  6. 6Which single pair should be added to R={(1,1),(2,2),(3,3),(1,2)} so that it becomes an equivalence relation on A={1,2,3}?
  7. 7On R, define aRb if and only if a−b is rational. What is the equivalence class of √2?
  8. 8Let R be an equivalence relation on A and let [a] and [b] denote equivalence classes. If aRb, which statement must be true?
  9. 9For the relation on A={1,2,3,4,5,6} defined by “a and b are both multiples of 3 or both are not multiples of 3”, which statement is true?
  10. 10For the relation defined by aRb iff a=b or both a and b are greater than 2 on A={1,2,3,4}, what is the equivalence class of 3?
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