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JEE Main & Advanced Sample Paper JEE Main Sample Paper-42

  • question_answer
    Directions: Question based on the following paragraph.
    If\[\cos \frac{\pi }{7},\cos \frac{3\pi }{7},\cos \frac{5\pi }{7}.\]are the roots of the equation \[8{{x}^{3}}-4{{x}^{b}}-4x+1=0\].
    Find the value of \[\sec \frac{\pi }{7}+\sec \frac{3\pi }{7}+\sec \frac{5\pi }{7}.\]

    A)  2                                

    B)  4

    C)  8                                

    D)  None of these

    Correct Answer: B

    Solution :

    Since, \[\sec \frac{\pi }{7},\sec \frac{3\pi }{7},\sec \frac{5\pi }{7}\] are the roots of Eq.(ii) \[\therefore \]\[\sec \frac{\pi }{7}+\sec \frac{3\pi }{7}+\sec \frac{5\pi }{7}=4\]


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