JEE Main & Advanced Mathematics Limits, Continuity and Differentiability Question Bank Mock Test - Continuity and Differentiability

  • question_answer
    \[f(x)=\underset{n\to \infty }{\mathop{\lim }}\,\frac{{{(x-1)}^{2n}}-1}{{{(x-1)}^{2n}}+1}\]is discontinuous at

    A) \[x=0\]only       

    B) \[x=2\]only

    C) \[x=0\]and 2     

    D) none of these

    Correct Answer: C

    Solution :

    [c] \[f(x)=\underset{n\to \infty }{\mathop{\lim }}\,\frac{{{[{{(x-1)}^{2}}]}^{n}}-1}{{{[{{(x-1)}^{2}}]}^{n}}+1}\] \[=\underset{n\to \infty }{\mathop{\lim }}\,\frac{1-\frac{1}{{{[{{(x-1)}^{2}}]}^{n}}}}{1+\frac{1}{{{[{{(x-1)}^{2}}]}^{n}}}}\] \[=\left\{ \begin{matrix}    -1,\,\,\,\,0\le {{(x-1)}^{2}}<1  \\    0,\,\,\,\,{{(x-1)}^{2}}=1  \\    1,\,\,\,\,\,{{(x-1)}^{2}}>1  \\ \end{matrix} \right.\] \[=\left\{ \begin{matrix}    1,\,\,\,\,\,\,x<0  \\    \begin{align}   & 0,\,\,\,\,\,\,\,\,\,x=0 \\  & -1,\,\,\,\,\,0<x<2 \\ \end{align}  \\    0,\,\,\,\,\,\,x=2  \\    1,\,\,\,\,\,\,x>2  \\ \end{matrix} \right.\] Thus, f(x) is discontinuous at x=0.2.


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