JEE Main & Advanced Mathematics Functions Question Bank Continuity

  • question_answer
    If the function \[f(x)=\left\{ \begin{align}   & {{(\cos x)}^{1/x}},\ x\ne 0 \\  & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,k,\,x=0 \\ \end{align} \right.\]is continuous at \[x=0\], then the value of k is                     [Kurukshetra CEE 1996]

    A)            1

    B)            ?1

    C)            0

    D)            e

    Correct Answer: A

    Solution :

               \[\underset{x\to 0}{\mathop{\lim }}\,\,\,{{(\cos x)}^{1/x}}=k\,\,\Rightarrow \,\,\underset{x\to 0}{\mathop{\lim }}\,\frac{1}{x}\log \,(\cos x)=\log k\]                    \[\Rightarrow \,\,\underset{x\to 0}{\mathop{\lim }}\,\,\,\frac{1}{x}\,\,\underset{x\to 0}{\mathop{\lim }}\,\,\,\log \,\cos x=\log k\]            \[\Rightarrow \,\,\,\underset{x\to 0}{\mathop{\lim }}\,\,\,\frac{1}{x}\times 0={{\log }_{e}}k\,\,\Rightarrow \,k=1\] .


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