JEE Main & Advanced Sample Paper JEE Main - Mock Test - 26

  • question_answer
    Let \[f(x)\] be a continuous function which satisfies \[f({{x}^{2}}+1)=\frac{2}{f({{2}^{x}})-1}\] and \[f(x)>0\,\forall \,x\,\in R.\]  Then \[\underset{x\to 1}{\mathop{\lim }}\,f(x)\] is

    A) \[0\]                 

    B)        \[1\]

    C) \[2\]                 

    D)        \[3\]

    Correct Answer: C

    Solution :

    [c] \[f({{x}^{2}}+1)=\frac{2}{f({{2}^{x}})-1}\] \[\Rightarrow \,\,\,\,\,\underset{x\to 0}{\mathop{\lim }}\,\,\,f({{x}^{2}}+1)=\underset{x\to 0}{\mathop{\lim }}\,\frac{2}{f({{2}^{x}})-1}\] \[\Rightarrow \,\,\,\,\,L=\frac{2}{L-1}\]   (where \[L=\underset{x\to 1}{\mathop{\lim }}\,\,\,f(x)\]) \[\Rightarrow \,\,\,\,\,{{L}^{2}}-L-2=0\] \[\Rightarrow \,\,\,\,\,(L-2)(L+1)=0\] \[\therefore \,\,\,\,\,\,\,\,L=2\,\,\,(\,\,\,f(x)>0)\]


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