CET Karnataka Engineering CET - Karnataka Engineering Solved Paper-2008

  • question_answer
    If   \[f({{x}^{5}})=5{{x}^{3}},\]then \[f'(x)\] is equal to

    A)  \[\frac{3}{\sqrt[5]{{{x}^{2}}}}\]                

    B)  \[\frac{3}{\sqrt[5]{x}}\]

    C)  \[\frac{3}{x}\]                                  

    D)  \[\sqrt[5]{x}\]

    Correct Answer: A

    Solution :

    Given,  \[f({{x}^{5}})=5{{x}^{3}}\] Let      \[{{x}^{5}}=y\Rightarrow {{x}^{3}}={{y}^{3/5}}\] \[\therefore \]  \[f(y)=5{{y}^{3/5}}\] or            \[f(x)=5{{x}^{3/5}}\] On differentiating w.r.t. x, we get                 \[f'(x)=5.\frac{3}{5}{{x}^{-2/5}}\]                 \[=\frac{3}{\sqrt[5]{{{x}^{2}}}}\]


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