JEE Main & Advanced Mathematics Functions Question Bank Limits

  • question_answer
    If \[f(x)=\left\{ \begin{align}   & x,\ \ \text{if }x\text{ is rational } \\  & -x,\ \text{if }x\text{ is irrational} \\ \end{align} \right.,\] then \[\underset{x\to 0}{\mathop{\lim }}\,f(x)\]is                 [Kurukshetra CEE 1998; UPSEAT 2004]

    A)                 Equal to 0

    B)                 Equal to 1

    C)                 Equal to ?1

    D)                 Indeterminate

    Correct Answer: A

    Solution :

                       \[\underset{x\to {{0}^{-}}}{\mathop{\lim }}\,f(x)=\underset{h\to 0}{\mathop{\lim }}\,f(0-h)=\underset{h\to 0}{\mathop{\lim }}\,f(0-h)=0\]            and \[\underset{x\to {{0}^{+}}}{\mathop{\lim }}\,f(x)=\underset{h\to 0}{\mathop{\lim }}\,f(0+h)=\underset{h\to 0}{\mathop{\lim }}\,\,\,\,-(0+h)=0\]                 \[\therefore \,\,\,\underset{x\to 0}{\mathop{\lim }}\,\,\,f(x)=0\], \[\left( \because \underset{x\to {{0}^{-}}}{\mathop{\lim }}\,f(x)=\underset{x\to {{0}^{+}}}{\mathop{\lim }}\,f(x) \right)\].


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