Class CBSE Class 12 Mathematics Relations and Functions Q #920
COMPETENCY BASED
APPLY
5 Marks 2023 LA
A relation $R$ is defined on a set of real numbers $\mathbb{R}$ as:$$R = \{(x, y) : x \cdot y \text{ is an irrational number}\}$$Check whether $R$ is reflexive, symmetric, and transitive or not.

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Step-by-Step Solution

Reflexive:

For $R$ to be reflexive, $(x, x) \in R$ for all $x \in \mathbb{R}$. This means $x \cdot x = x^2$ must be irrational for all real numbers $x$. However, if $x = 1$, then $x^2 = 1$, which is rational. Therefore, $R$ is not reflexive.

Symmetric:

For $R$ to be symmetric, if $(x, y) \in R$, then $(y, x) \in R$. This means if $x \cdot y$ is irrational, then $y \cdot x$ must also be irrational. Since $x \cdot y = y \cdot x$, if $x \cdot y$ is irrational, then $y \cdot x$ is also irrational. Therefore, $R$ is symmetric.

Transitive:

For $R$ to be transitive, if $(x, y) \in R$ and $(y, z) \in R$, then $(x, z) \in R$. This means if $x \cdot y$ is irrational and $y \cdot z$ is irrational, then $x \cdot z$ must be irrational. Let's consider a counterexample: Let $x = \sqrt{2}$, $y = \sqrt{2}$, and $z = \sqrt{2}$. Then $x \cdot y = \sqrt{2} \cdot \sqrt{2} = 2$, which is rational. So, $(x, y) \notin R$. Let $x = \sqrt{2}$, $y = \sqrt{3}$, and $z = \sqrt{2}$. Then $x \cdot y = \sqrt{6}$ (irrational) and $y \cdot z = \sqrt{6}$ (irrational). But $x \cdot z = \sqrt{2} \cdot \sqrt{2} = 2$ (rational). Thus, $(x, z) \notin R$. Another counterexample: Let $x = \sqrt{2}$, $y = 1/\sqrt{2}$, and $z = \sqrt{3}$. $x \cdot y = \sqrt{2} \cdot (1/\sqrt{2}) = 1$ (rational), so $(x, y) \notin R$. Let $x = \sqrt{2}$, $y = \sqrt{2}$, $z = \sqrt{3}$. Consider $x = \sqrt{2}$, $y = \sqrt{3}$, $z = \sqrt{2}$. $xy = \sqrt{6}$ (irrational), $yz = \sqrt{6}$ (irrational), but $xz = 2$ (rational). Thus, $R$ is not transitive.

Correct Answer: Reflexive: No, Symmetric: Yes, Transitive: No

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because the student needs to apply the definitions of reflexive, symmetric, and transitive relations to a specific relation defined on real numbers to determine its properties.
Knowledge Dimension: CONCEPTUAL
Justification: The question requires understanding the concepts of reflexive, symmetric, and transitive relations and applying them to a specific relation defined using irrational numbers. It's not just recalling definitions but applying them in a non-trivial context.
Syllabus Audit: In the context of CBSE Class 12, this is classified as COMPETENCY. The question assesses the student's ability to apply the definitions of relations (reflexive, symmetric, transitive) to a specific case, requiring more than just recalling the definitions from the textbook. It involves logical reasoning and application of concepts.