JEE Main & Advanced Mathematics Differentiation Question Bank Differentiation by Substitution

  • question_answer
    If \[y={{\sin }^{-1}}\frac{\sqrt{(1+x)}+\sqrt{(1-x)}}{2}\], then \[\frac{dy}{dx}=\]

    A)            \[\frac{1}{\sqrt{(1-{{x}^{2}})}}\]

    B)            \[-\frac{1}{\sqrt{(1-{{x}^{2}})}}\]

    C)            \[-\frac{1}{2\sqrt{(1-{{x}^{2}})}}\]

    D)            None of these

    Correct Answer: C

    Solution :

               Put \[x=\cos \theta \]            \[1+\cos \theta =2{{\cos }^{2}}\left( \frac{\theta }{2} \right)\,,\ \ 1-\cos \theta =2{{\sin }^{2}}\left( \frac{\theta }{2} \right)\]            \[\therefore y={{\sin }^{-1}}\left[ \frac{1}{\sqrt{2}}\left\{ \cos \left( \frac{\theta }{2} \right)+\sin \left( \frac{\theta }{2} \right) \right\} \right]\]                \[={{\sin }^{-1}}\sin \left( \frac{\theta }{2}+\frac{\pi }{4} \right)=\frac{\theta }{2}+\frac{\pi }{4}\]            Þ \[y=\frac{1}{2}{{\cos }^{-1}}x+\frac{\pi }{4}\Rightarrow \frac{dy}{dx}=\frac{-1}{2\sqrt{1-{{x}^{2}}}}\].


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