JEE Main & Advanced Mathematics Differentiation Question Bank Differentiation by Substitution

  • question_answer
    If \[y={{\cos }^{-1}}\left( \frac{3\cos x+4\sin x}{5} \right)\], then \[\frac{dy}{dx}=\]

    A)            0

    B)            1

    C)            \[-1\]

    D)            \[\frac{1}{2}\]

    Correct Answer: B

    Solution :

               \[y={{\cos }^{-1}}\left[ \frac{3}{5}\cos x-\frac{4}{5}\sin x \right]\]            Putting \[\frac{3}{5}=r\cos \theta ,\,\,\frac{4}{5}=r\sin \theta \Rightarrow r=1\]                    Þ \[y={{\cos }^{-1}}[\cos \theta \cos x-\sin \theta \sin x]=\theta +x\]\[\Rightarrow \frac{dy}{dx}=1\].


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