KVPY Sample Paper KVPY Stream-SX Model Paper-28

  • question_answer
    On the set N of all natural numbers, define the relation R by a R b, iff GCD of a and b is 2. Then, R is

    A) Reflexive, but not symmetric

    B) Symmetric only

    C) Reflexive and transitive

    D) Not reflexive, not symmetric, not transitive

    Correct Answer: B

    Solution :

    \[\operatorname{a} R a\], then GCD of a and a is a.
    \[\therefore \]R is not reflexive.
    Now, \[\operatorname{aRb}\Rightarrow bRa\]
    If GCD of a and b is 2, then GCD of band a is 2.
    \[\therefore \]R is symmetric.
    Now. \[\operatorname{aRb},bRc\,aRc\]
    If GCD of a and b is 2 and GCD of b and c is
    2, then it need not be GCD of a and c is
    2. \[\therefore \]R is not transitive.
    e.g.,\[6\operatorname{R}2,2\operatorname{R}12\operatorname{but}\text{ }612.\]


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