JEE Main & Advanced Sample Paper JEE Main - Mock Test - 27

  • question_answer
    The function \[\operatorname{f}(x) = {{[x]}^{2}}- [{{x}^{2}}]\] (where [y] is the greatest integer less than or equal to y), is discontinuous at

    A) All integ ers

    B)   All integers except 0 and 1

    C)   All integers except 0

    D)   All integers except 1

    Correct Answer: C

    Solution :

    \[f(x)={{[x]}^{2}}-[{{x}^{2}}]\]
    Check continuity at \[\operatorname{x} = 0\]
    \[\underset{x\to {{0}^{+}}}{\mathop{\lim }}\, f(x)=\,\,\underset{x\to {{0}^{+}}}{\mathop{\lim }}\,\,{{\left[ x \right]}^{2}}-\,\,[{{x}^{2}}]=\,\,0\]
    \[\underset{x\to {{0}^{-}}}{\mathop{\lim }}\, f(x)=\,\,\underset{x\to {{0}^{-}}}{\mathop{\lim }}\,\,{{\left[ x \right]}^{2}}-\,\,[{{x}^{2}}]\]
    \[=\,\,\,{{(-1)}^{2}}-0=1\]
    Thus, discontinuous at \[\operatorname{x} = 0\]
    Check continuity at \[\operatorname{x} = 1\]
    \[\underset{x\to {{1}^{+}}}{\mathop{\lim }}\, f(x)=\,\,1-1=0\]
    \[\underset{x\to {{1}^{-}}}{\mathop{\lim }}\, f(x)=\,\,0-0=0\]
    Also \[\operatorname{f}(1)=0\]
    Hence continuous at \[\operatorname{x} = 1.\]


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