RAJASTHAN ­ PET Rajasthan PET Solved Paper-2009

  • question_answer
    If function\[f(x),x\in R\]is differentiable and \[f(1)=1,\]then value of\[\underset{x\to 0}{\mathop{\lim }}\,\frac{(1-\cos 2x)}{{{x}^{2}}}\]is

    A)  0                 

    B)  1

    C)  \[f'(1)\]            

    D)  \[\infty \]

    Correct Answer: C

    Solution :

     \[\underset{x\to 1}{\mathop{\lim }}\,\int_{1}^{f(x)}{\frac{t\,dt}{(x-1)}}=\underset{x\to 1}{\mathop{\lim }}\,=\frac{1}{x-1}\left[ \left( \frac{{{t}^{2}}}{2} \right) \right]_{1}^{f(x)}\] \[=\frac{1}{2}\underset{x\to 1}{\mathop{\lim }}\,\frac{1}{x-1}[{{\{f(x)\}}^{2}}-1]\] \[=\frac{1}{2}\underset{x\to 1}{\mathop{\lim }}\,\frac{[2f(x)f'(x)]}{1}\] [by L' Hospital rule]


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