Easy MCQ +4 / -1 PYQ · JEE Mains 2022

Let $R_{1}$ and $R_{2}$ be two relations defined on $\mathbb{R}$ by

$a \,R_{1} \,b \Leftrightarrow a b \geq 0$ and $a \,R_{2} \,b \Leftrightarrow a \geq b$

Then,

  1. A $R_{1}$ is an equivalence relation but not $R_{2}$
  2. B $R_{2}$ is an equivalence relation but not $R_{1}$
  3. C both $R_{1}$ and $R_{2}$ are equivalence relations
  4. D neither $R_{1}$ nor $R_{2}$ is an equivalence relation Correct answer

Solution

<p>$a\,{R_1}\,b \Leftrightarrow ab \ge 0$</p> <p>So, definitely $(a,a) \in {R_1}$ as ${a^2} \ge 0$</p> <p>If $(a,b) \in {R_1} \Rightarrow (b,a) \in {R_1}$</p> <p>But if $(a,b) \in {R_1},(b,c) \in {R_1}$</p> <p>$\Rightarrow$ Then $(a,c)$ may or may not belong to R<sub>1</sub></p> <p>{Consider $a = - 5,b = 0,c = 5$ so $(a,b)$ and $(b,c) \in {R_1}$ but $ac < 0$}</p> <p>So, R<sub>1</sub> is not equivalence relation</p> <p>$a\,{R_2}\,b \Leftrightarrow a \ge b$</p> <p>$(a,a) \in {R_2} \Rightarrow$ so reflexive relation</p> <p>If $(a,b) \in {R_2}$ then $(b,a)$ may or may not belong to R<sub>2</sub></p> <p>$\Rightarrow$ So not symmetric</p> <p>Hence it is not equivalence relation</p>

About this question

Subject: Mathematics · Chapter: Sets, Relations and Functions · Topic: Sets and Operations

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