J & K CET Engineering J and K - CET Engineering Solved Paper-2005

  • question_answer
    If \[f(x)=\left\{ \begin{matrix}    x, & for & <x<1  \\    2-x, & for & 1\le x<2  \\    x-(1/2){{x}^{2}}, & for & x=2  \\ \end{matrix} \right.\] Then, \[f'(1)\]is equal to

    A)  \[-1\]            

    B)  \[1\]

    C)  \[0\]            

    D)  None of these

    Correct Answer: D

    Solution :

    Given,  \[f(x)=\left\{ \begin{matrix}    x, & if & 0<x<1  \\    2-x, & if & 1\le x<2  \\    x-\frac{1}{2}{{x}^{2}}, & if & x=2  \\ \end{matrix} \right.\] At    \[x=1,\] \[RHD=\underset{h\to {{0}^{+}}}{\mathop{\lim }}\,\frac{f(1+h)-f(1)}{h}\] \[=\underset{h\to 0}{\mathop{\lim }}\,\frac{2-(1+h)-(2-1)}{h}\] \[=\underset{h\to 0}{\mathop{\lim }}\,\frac{2-1-h-1}{h}=-1\] \[LHD=\underset{h\to {{0}^{-}}}{\mathop{\lim }}\,\frac{f(1-h)-f(1)}{-h}\] \[=\underset{h\to 0}{\mathop{\lim }}\,\frac{(1-h)-(2-1)}{-h}=1\] \[\therefore \] \[LHD\ne RHD\]


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