JEE Main & Advanced Mathematics Definite Integration Question Bank Fundamental definite integration, Definite integration by substitution

  • question_answer
    If \[\int_{0}^{k}{\frac{dx}{2+8{{x}^{2}}}}=\frac{\pi }{16}\,,\] then \[k=\]

    A)                 1             

    B)                 \[\frac{1}{2}\]

    C)                 \[\frac{1}{4}\]   

    D)                 None of these

    Correct Answer: B

    Solution :

    \[\int_{0}^{k}{\frac{1}{2+8{{x}^{2}}}dx=\frac{1}{2}\int_{0}^{k}{\frac{dx}{1+{{(2x)}^{2}}}=\frac{1}{4}\int_{0}^{2k}{\frac{dt}{1+{{t}^{2}}}}}}\]                    \[=\frac{1}{4}|{{\tan }^{-1}}t|_{0}^{2k}=\frac{1}{4}{{\tan }^{-1}}2k\].                                 Comparing it with the given value, we get \[{{\tan }^{-1}}2k=\frac{\pi }{4}\Rightarrow 2k=1\Rightarrow k=\frac{1}{2}\].


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