JEE Main & Advanced Mathematics Functions Question Bank Differentiability

  • question_answer
    sThe function \[f(x)=\left\{ \begin{matrix}    {{e}^{2x}}-1 & , & x\le 0  \\    ax+\frac{b{{x}^{2}}}{2}-1 & , & x>0  \\ \end{matrix} \right.\] is continuous and differentiable for [AMU 2002]

    A)            \[[.]\]

    B)            \[a=2,\,b=4\]

    C)            \[a=2,\,\]any \[b\]

    D)            Any \[a,\,\,\,b=4\]

    Correct Answer: C

    Solution :

               \[\because f\] is continuous at \[x=0\], \\[f({{0}^{-}})=f({{0}^{+}})=f(0)=-1\]            Also \[L{f}'(0)=R{f}'(0)\]            Þ \[\underset{h\to 0}{\mathop{\lim }}\,\frac{f(0-h)-f(0)}{-h}=\underset{h\to 0}{\mathop{\lim }}\,\frac{f(0+h)-f(0)}{h}\]            Þ \[\underset{h\to 0}{\mathop{\lim }}\,\left( \frac{{{e}^{-2h}}-1+1}{-h} \right)=\underset{h\to 0}{\mathop{\lim }}\,\left( \frac{ah+\frac{b{{h}^{2}}}{2}-1+1}{h} \right)\]            Þ \[\underset{h\to 0}{\mathop{\lim }}\,\left( \frac{-2{{e}^{-2h}}}{-1} \right)=\underset{h\to 0}{\mathop{\lim }}\,\left( a+\frac{bh}{2} \right)\]            Þ \[2=a+0\] Þ \[a=2,\,\,b\] any number.


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