Hard MCQ +4 / -1 PYQ · JEE Mains 2020

Let R1 and R2 be two relation defined as follows :
R1 = {(a, b) $\in$ R2 : a2 + b2 $\in$ Q} and
R2 = {(a, b) $\in$ R2 : a2 + b2 $\notin$ Q},
where Q is the set of all rational numbers. Then :

  1. A Neither R<sub>1</sub> nor R<sub>2</sub> is transitive. Correct answer
  2. B R<sub>2</sub> is transitive but R<sub>1</sub> is not transitive.
  3. C R<sub>1</sub> and R<sub>2</sub> are both transitive.
  4. D R<sub>1</sub> is transitive but R<sub>2</sub> is not transitive.

Solution

For R<sub>1</sub> :<br><br>Let a = 1 + $\sqrt 2$, b = 1 $-$ $\sqrt 2$, c = ${8^{{1 \over 4}}}$<br><br>aR<sub>1</sub>b : a<sup>2</sup> + b<sup>2</sup> = 6 $\in$ Q<br><br>bR<sub>1</sub>c : b<sup>2</sup> + c<sup>2</sup> = 3 $-$ 2$\sqrt 2$ + 2$\sqrt 2$ = 3 $\in$ Q<br><br>aR<sub>1</sub>c : a<sup>2</sup> + c<sup>2</sup> = 3 + 2$\sqrt 2$ + 2$\sqrt 2$ $\notin$ Q<br><br>$\therefore$ R<sub>1</sub> is not transitive.<br><br>For R<sub>2</sub> : <br><br>Let a = 1 + $\sqrt 2$, b = $\sqrt 2$, c = 1 $-$ $\sqrt 2$<br><br>aR<sub>2</sub>b : a<sup>2</sup> + b<sup>2</sup> = 5 + 2$\sqrt 2$ $\notin$ Q<br><br>bR<sub>2</sub>c : b<sup>2</sup> + c<sup>2</sup> = 5 $-$ 2$\sqrt 2$ $\notin$ Q<br><br>aR<sub>2</sub>c : a<sup>2</sup> + c<sup>2</sup> = 3 + 2$\sqrt 2$ + 3 $-$ 2$\sqrt 2$ = 6 $\in$ Q<br><br>$\therefore$ R<sub>2</sub> is not transitive.

About this question

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

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