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JEE Main & Advanced Mathematics Trigonometric Identities Question Bank Trigonometrical ratios of multiple and sub multiple angles

  • question_answer
    If \[\cos 3\theta =\alpha \cos \theta +\beta {{\cos }^{3}}\theta ,\]then \[(\alpha ,\beta )=\]

    A) \[(3,\,4)\]

    B) \[(4,\,3)\]

    C) \[(-3,\,4)\]

    D) \[(3,\,-4)\]

    Correct Answer: C

    Solution :

    Given that \[\cos 3\theta =\alpha \cos \theta +\beta {{\cos }^{3}}\theta \] But \[\cos 3\theta =4{{\cos }^{3}}\theta -3\cos \theta \] Þ  \[(\alpha ,\beta )=(-3,\,4)\].


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