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

  • question_answer
    Let \[f:R\to R\] be a function defined by f(x) max\[\{x,\text{ }{{x}^{3}}\}\]. The set of all points where f(x) is NOT differentiable is

    A) {-1, 1}

    B) {-1, 0}

    C) {0, 1}

    D) {-1, 0, 1}

    Correct Answer: D

    Solution :

    [d] \[f(x)=\max .\,\,\left\{ x,{{x}^{3}} \right\}\] \[=\left\{ \begin{matrix}    x; & x<-1  \\    {{x}^{3}}; & -1\le x\le 0  \\    x; & 0\le x\le 1  \\    {{x}^{3}}; & x\ge 1  \\ \end{matrix} \right.\] \[\therefore f'(x)=\left\{ \begin{matrix}    1; & x<-1  \\    3{{x}^{2}}; & -1\le x\le 0  \\    1; & 0\le x\le 1  \\    3{{x}^{2}}; & x\ge 1  \\ \end{matrix} \right.\] Clearly f is not differentiable at -1, 0 and 1.


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