JEE Main & Advanced Mathematics Functions Question Bank Functions

  • question_answer
    The domain of the derivative of the function \[f(x)=\left\{ \begin{align}   & {{\tan }^{-1}}x\ \ \ \ \ ,\ |x|\ \le 1 \\  & \frac{1}{2}(|x|\ -1)\ ,\ |x|\ >1 \\ \end{align} \right.\] is [IIT Screening 2002]

    A)                    \[R-\{0\}\]

    B)            \[R-\{1\}\]

    C)            \[R-\{-1\}\]

    D)            \[R-\{-1,\ 1\}\]

    Correct Answer: C

    Solution :

               \[f(x)=\left\{ \begin{align}   & \frac{1}{2}(-x-1),\,\,\,x<-1 \\  & {{\tan }^{-1}}x,\,\,\,\,-1\le x\le 1 \\  & \frac{1}{2}(x+1),\,\,\,x>1 \\ \end{align} \right.\]; \[{f}'(x)=\left\{ \begin{align}   & -\frac{1}{2},\,\,\,\,\,\,\,\,x<-1 \\  & \frac{1}{1+{{x}^{2}}},\,\,-1<x<1 \\  & \frac{1}{2},\,\,\,\,\,\,\,\,\,\,\,\,x>1 \\ \end{align} \right.\]            \[{f}'(-1-0)=-\frac{1}{2};\,\,{f}'(-1+0)=\frac{1}{1+{{(-1+0)}^{2}}}=\frac{1}{2}\]            \[{f}'(1-0)=\frac{1}{1+{{(1-0)}^{2}}}=\frac{1}{2};\,\,{f}'(1+0)=\frac{1}{2}\]            \\[{f}'(-1)\] Does not exist; \ domain of \[{f}'(x)=R-\{-1\}\].


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