CBSE Class 12 Mathematics Relations and Functions Q #2106
COMPETENCY BASED
APPLY
2 Marks VSA
How many equivalence relations on the set {1,2,4} containing (1,2) and (2,1) are there in all? Justify your answer.

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Detailed Solution

Step 1: Understanding Equivalence Relations

An equivalence relation $R$ on set $A = \{1, 2, 4\}$ must be reflexive, symmetric, and transitive. Reflexivity requires $(1,1), (2,2), (4,4) \in R$.

Step 2: Incorporating Given Elements

We are given $(1,2) \in R$ and $(2,1) \in R$. By symmetry, these are already satisfied. By transitivity, since $(1,2) \in R$ and $(2,1) \in R$, we must have $(1,1) \in R$ and $(2,2) \in R$, which are already covered by reflexivity.

Step 3: Analyzing Partitions

Equivalence relations correspond to partitions of the set. The element $4$ can either be in a singleton set $\{4\}$ or grouped with $\{1, 2\}$.

Step 4: Evaluating Possible Partitions

Case 1: Partition is $\{\{1, 2\}, \{4\}\}$. The relation is $R_1 = \{(1,1), (2,2), (4,4), (1,2), (2,1)\}$. This satisfies all conditions.
Case 2: Partition is $\{\{1, 2, 4\}\}$. The relation is the universal relation $R_2 = A \times A$. This contains $(1,2)$ and $(2,1)$ and is an equivalence relation.

Final Answer: 2

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because it requires the student to apply the definitions of equivalence relations and partition theory to a specific set constraint.
Knowledge Dimension: PROCEDURAL
Justification: The student must follow a logical sequence of checking properties (reflexive, symmetric, transitive) to determine valid relations.
Syllabus Audit: In the context of CBSE Class 12, this is classified as COMPETENCY. It tests the conceptual depth of Relations and Functions beyond rote memorization of definitions.
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