JEE Main & Advanced Mathematics Functions Question Bank Differentiability

  • question_answer
    Let \[f(x)\,=\left\{ \begin{matrix}    x+1, & \text{when} & x<2  \\    2x-1, & \text{when} & x\ge 2  \\ \end{matrix} \right.\,,\,\] then \[{f}'(2)=\] [Karnataka CET 2002]

    A)            0

    B)            1

    C)            2

    D)            Does not exist

    Correct Answer: D

    Solution :

               \[R{f}'(2)\]\[=\underset{h\to 0}{\mathop{\text{lim}}}\,\frac{f(2+h)-f(2)}{h}\]            \[=\underset{h\to 0}{\mathop{\text{lim}}}\,\frac{2(2+h)-1-(4-1)}{h}\]\[=\underset{h\to 0}{\mathop{\text{lim}}}\,\frac{4+2h-1-3}{h}=2\]            and \[L{f}'(2)=\underset{h\to 0}{\mathop{\text{lim}}}\,\frac{f(2-h)-f(2)}{-h}=\underset{h\to 0}{\mathop{\text{lim}}}\,\frac{2-h+1-3}{-h}=1\].            Thus \[{f}'(2)\] does not exist.


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