JEE Main & Advanced Mathematics Differentiation Question Bank Differentiation by Substitution

  • question_answer
    The differential coefficient of \[{{\tan }^{-1}}\frac{2x}{1-{{x}^{2}}}\] w.r.t.  \[{{\sin }^{-1}}\frac{2x}{1+{{x}^{2}}}\] is  [Roorkee 1966; BIT Ranchi 1996; Karnataka CET 1994; MP PET 1999; UPSEAT 1999, 2001]

    A)            1

    B)            ? 1

    C)            0

    D)            None of these

    Correct Answer: A

    Solution :

               Let \[{{y}_{1}}={{\tan }^{-1}}\frac{2x}{1-{{x}^{2}}}\] and \[{{y}_{2}}={{\sin }^{-1}}\frac{2x}{1+{{x}^{2}}}\]            Differentiating w.r.t. x of\[{{y}_{1}}\]and \[{{y}_{2}}\], we get                    \[\frac{d{{y}_{1}}}{dx}=\frac{d}{dx}\left[ {{\tan }^{-1}}\frac{2x}{1-{{x}^{2}}} \right]\]            Putting \[x=\tan \theta \],  \[\therefore {{y}_{1}}={{\tan }^{-1}}\tan 2\theta =2\theta =2{{\tan }^{-1}}x\]            and \[{{y}_{2}}={{\sin }^{-1}}\sin 2\theta =2{{\tan }^{-1}}x\]            Again \[\frac{d{{y}_{1}}}{dx}=\frac{d}{dx}[2{{\tan }^{-1}}x]=\frac{2}{1+{{x}^{2}}}\]               .....(i)            and \[\frac{d{{y}_{2}}}{dx}=\frac{d}{dx}[2{{\tan }^{-1}}x]=\frac{2}{1+{{x}^{2}}}\]                   .....(ii)            Hence \[\frac{d{{y}_{1}}}{d{{y}_{2}}}=1\].


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