JEE Main & Advanced Mathematics Differentiation Question Bank Differentiation by Substitution

  • question_answer
    Derivative of \[{{\sec }^{-1}}\left\{ \frac{1}{2{{x}^{2}}-1} \right\}\]w.r.t \[\sqrt{1+3x}\]at \[x=-\frac{1}{3}\] is                          [EAMCET 1991]

    A)            0

    B)            1/2

    C)            1/3

    D)            None of these

    Correct Answer: A

    Solution :

               \[{{\sec }^{-1}}\frac{1}{(2{{x}^{2}}-1)}=2{{\cos }^{-1}}x\]                    \[\therefore y=2{{\cos }^{-1}}x,\ \ z=\sqrt{1+3x}\]     \[\frac{dy}{dz}=\frac{dy}{dx}\div \frac{dz}{dx}=-\frac{2}{\sqrt{1-{{x}^{2}}}}.\frac{2\sqrt{1+3x}}{3}=0,\,\left( \text{at}\ \ \ x=-\frac{1}{3} \right)\].


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