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NDA 2025 Maths · Mathematics · Sets, Relations and Functions · medium

Relations on the set of all straight lines in a plane: 1. "Is parallel to" (a line being parallel to itself) is an equivalence relation. 2. "Is…

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Consider the following statements about relations on the set of all straight lines in a plane:
1. "Is parallel to" (a line being parallel to itself) is an equivalence relation.
2. "Is perpendicular to" is a transitive relation.
Which of the statements is/are correct?
  1. A. 1 only
  2. B. 2 only
  3. C. Both 1 and 2
  4. D. Neither 1 nor 2
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Answer: (A) 1 only

Parallelism is reflexive (by convention), symmetric and transitive, so statement 1 is true. Perpendicularity is symmetric but not transitive: if l⊥ml\perp m and m⊥nm\perp n then l∥nl\parallel n, not l⊥nl\perp n. So statement 2 is false.
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