JEE Main & Advanced Mathematics Functions Question Bank Differentiability

  • question_answer
    The value of m for which the function                                                                                   \[f(x)=\left\{ \begin{align}   & m{{x}^{2}},\,x\le 1 \\  & \,\,\,\,2x,\,x>1 \\ \end{align} \right.\]  is differentiable at                                                                                                   \[x=1\] ,is [MP PET 1998]

    A)            0

    B)            1

    C)            2

    D)            Does not exist

    Correct Answer: D

    Solution :

                                                          \[L\,{f}'\,(1)=\underset{h\to 0}{\mathop{\lim }}\,\,\,\frac{f(1-h)-f(1)}{-h}\]                                                          \[=\underset{h\to 0}{\mathop{\lim }}\,\,\,\frac{m\,{{(1-h)}^{2}}-m}{-h}\]                                           \[=\underset{h\to 0}{\mathop{\lim }}\,\,\,\frac{m\,[1+{{h}^{2}}-2h-1]}{-h}\]                                                                       \[=\underset{h\to 0}{\mathop{\lim }}\,\,m\,(2-h)=2m\]  and                                           \[R\,{f}'\,(1)=\underset{h\to 0}{\mathop{\lim }}\,\,\,\frac{f\,(1+h)-f(1)}{h}\]                                    \[=\underset{h\to 0}{\mathop{\lim }}\,\,\frac{2\,(1+h)-m}{h}\]. For differentiability,                                                                                     \[L\,{f}'(1)=R\,{f}'\,(1)\] .                    But for any value of                                                                               \[m,\,\,R\,\,{f}'\,(1)=L\,{f}'(1)\]  not possible.


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