JEE Main & Advanced Mathematics Trigonometric Identities Question Bank Fundamental trigonometrical ratios and functions, Trigonometrical ratio of allied angles

  • question_answer
    If \[(\sec \alpha +\tan \alpha )(\sec \beta +\tan \beta )(\sec \gamma +\tan \gamma )\]\[=\tan \alpha \tan \beta \tan \gamma \], then \[(\sec \alpha -\tan \alpha )(\sec \beta -\tan \beta )\] \[(\sec \gamma -\tan \gamma )=\]        [Kurukshetra CEE 1998]

    A) \[\cot \alpha \cot \beta \cot \gamma \]

    B) \[\tan \alpha \tan \beta \tan \gamma \]

    C) \[\cot \alpha +\cot \beta +\cot \gamma \]

    D) \[\tan \alpha +\tan \beta +\tan \gamma \]

    Correct Answer: A

    Solution :

    Given: \[(\sec \alpha +\tan \alpha )(\sec \beta +\tan \beta )(\sec \gamma +\tan \gamma )\] \[=\tan \alpha \tan \beta \tan \gamma \]           ...(i) Let\[x=(\sec \alpha -\tan \alpha )(\sec \beta -\tan \beta )(\sec \gamma -\tan \gamma )\] ...(ii) Multiply both equations,  (i) and (ii), we get \[({{\sec }^{2}}\alpha -{{\tan }^{2}}\alpha )({{\sec }^{2}}\beta -{{\tan }^{2}}\beta )({{\sec }^{2}}\gamma -{{\tan }^{2}}\gamma )\] \[=x.(\tan \alpha \tan \beta \tan \gamma )\] \[\Rightarrow x=\frac{1}{\tan \alpha \tan \beta \tan \gamma }\] \[\therefore x=\cot \alpha \cot \beta \cot \gamma \]


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