JEE Main & Advanced Mathematics Functions Question Bank Differentiability

  • question_answer
    A function \[f(x)\,=\left\{ \begin{matrix}    1+x, & x\le 2  \\    5-x, & x>2  \\ \end{matrix} \right.\,\] is [AMU 2001]

    A)            Not continuous at \[x=2\]

    B)            Differentiable at \[x=2\]

    C)            Continuous but not differentiable at \[x=2\]

    D)            None of these

    Correct Answer: C

    Solution :

               \[\underset{h\to {{0}^{-}}}{\mathop{\lim }}\,1+(2-h)=3\], \[\underset{h\to {{0}^{+}}}{\mathop{\lim }}\,5-(2+h)=3\], \[f(2)=3\]            Hence, f is continuous at \[x=2\]            Now \[R{f}'(x)=\underset{h\to 0}{\mathop{\lim }}\,\frac{5-(2+h)-3}{h}=-1\]            \[L{f}'(x)=\underset{h\to 0}{\mathop{\lim }}\,\frac{1+(2-h)-3}{-h}=1\]            \[\because R{f}'(x)\ne L{f}'(x)\];  \ f is not differentiable at \[x=2\].


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