Class JEE Mathematics Sets, Relations, and Functions Q #1074
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4 Marks 2008 AIEEE MCQ SINGLE
Let $R$ be the real line. Consider the following subsets of the plane $R \times R$: $S = {(x, y) : y=x+1 ext{ and } 0
(A) Neither S nor T is an equivalence relation on R
(B) Both S and T are equivalence relation on R
(C) S is an equivalence relation on R but T is not
(D) T is an equivalence relation on R but S is not
Correct Answer: D
Explanation
Given $S = {(x, y) : y = x + 1 ext{ and } 0 < x < 2}$. Since $x \ne x+1$ for any $x \in (0,2)$, we have $(x, x) \notin S$. Therefore, $S$ is not reflexive, and hence not an equivalence relation. Also $T = {(x, y) : x - y ext{ is an integer }}$. Since $x - x = 0$ is an integer for all $x \in R$, $T$ is reflexive. If $x - y$ is an integer, then $y - x$ is also an integer, so $T$ is symmetric. If $x - y$ is an integer and $y - z$ is an integer, then $(x - y) + (y - z) = x - z$ is also an integer. Therefore, $T$ is transitive. Hence $T$ is an equivalence relation.