JEE Main & Advanced Mathematics Differentiation Question Bank Derivative at a point Standard Differentiation

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     If \[y={{\tan }^{-1}}\left( \frac{a\cos x-b\sin x}{b\cos x+a\sin x} \right)\] then \[\frac{dy}{dx}=\] [Kerala (Engg.) 2005]

    A)            2

    B)            ? 1

    C)            \[\frac{a}{b}\]

    D)            0

    E)                 \[\frac{b}{a}\]

    Correct Answer: B

    Solution :

               \[y={{\tan }^{-1}}\left( \frac{a\cos x-b\sin x}{b\cos x+a\sin x} \right)\]            Let \[a=r\sin \theta \]and \[b=r\cos \theta \]            \  \[y={{\tan }^{-1}}\left[ \frac{r\sin (\theta -x)}{r\cos (\theta -x)} \right]\]                    \[y=\theta -x\];\[y={{\tan }^{-1}}\left( \frac{a}{b} \right)-x\]                    \[\frac{dy}{dx}=-1\].


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