JEE Main & Advanced Mathematics Functions Question Bank Differentiability

  • question_answer
    Let \[f(x)=\left\{ \begin{align}   & 1\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\forall x<0 \\  & 1+\sin x\,\,\,\forall 0\le x\le \pi /2 \\ \end{align} \right.\], then what is the value of \[f'(x)\] at \[x=0\] [Orissa JEE 2005]

    A)            1

    B)            ?1

    C)            \[\infty \]

    D)            does not exist

    Correct Answer: D

    Solution :

               \[f(x)=\left\{ \begin{align}   & \,\,\,1\,\,\,\,\,\,\,\,\,\,\,\forall x<0 \\  & 1+\sin ,\,\,\,\forall \,0\le x<\frac{\pi }{2} \\ \end{align} \right.\]                    \[\therefore \,\,f'(x)=\left\{ \begin{align}   & \,\,\,0,\,\,\,\,\forall \,x<0\,(\text{LHD}) \\  & \cos x,\,\,0\le x\le \pi /2,\,\,(\text{RHD}) \\ \end{align} \right.\]                    \\[f'(0)=\left\{ \begin{align}   & \,\,0\,\,\,\,,\,\,x<0 \\  & \cos 0=1 \\ \end{align} \right.\], \\[f'(0)\] does not exist.


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