JEE Main & Advanced JEE Main Paper (Held On 12 April 2014)

  • question_answer
    Let \[f,g:R\to R\]be two functions defined by \[f(x)=\left\{ \begin{align}   & x\sin \left( \frac{1}{x} \right),x\ne 0 \\  & 0,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,,x=0 \\ \end{align} \right.,\]and \[g(x)=xf(x)\] Statement I: f is a continuous function at x = 0. Statement II: g is a differentiable function at x = 0.   [JEE Main Online Paper ( Held On 12 Apirl  2014 )

    A) Both statement I and II are false.

    B) Both statement I and II are true.

    C) Statement I is true, statement II is false.

    D) Statement I is false, statement II is true.

    Correct Answer: B

    Solution :

                     \[f(x)=\left\{ \begin{matrix}    x\sin \left( \frac{1}{x} \right),x\ne 0 & x\ne 0  \\    0, & x=0  \\ \end{matrix} \right.\]and \[g(x)=xf(x)\] LHL\[=\underset{h\to {{0}^{-}}}{\mathop{\lim }}\,\left\{ -h\sin \left( -\frac{1}{h} \right) \right\}\] = 0 × a finite quantity between ?1 and 1 = 0 \[RHL=\underset{h\to {{0}^{+}}}{\mathop{\lim }}\,h\sin \frac{1}{h}=0\] Also, x = 0 Thus LHL = RHL = f(0) \[\therefore \]f (x) is continuous at x = 0 \[g(x)=\left\{ \begin{matrix}    {{x}^{2}}\sin \frac{1}{x}, & x\ne 0  \\    0, & x=0  \\ \end{matrix} \right.\] For g(x) LHL \[=\underset{h\to {{o}^{-}}}{\mathop{\lim }}\,\left\{ -{{h}^{2}}\sin \left( \frac{1}{h} \right) \right\}\] \[={{0}^{2}}\times a\]a finite quantity between ?1 and 1 = 0 RHL\[=\underset{h\to {{0}^{+}}}{\mathop{\lim }}\,{{h}^{2}}\sin \left( \frac{1}{h} \right)=0\] Also g(0) = 0 \[\therefore \] (x) is continuous at x = 0


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