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    Question

    If cosx/siny = n and cosx/cosy = m, then the value of

    cos 2 y is: 1. m 2 /(m 2  + n 2 )2. 1/(m 2  + n 2 )3. n 2 /(m 2  + n 2 )4. 0
    A 1 Correct Answer Incorrect Answer
    B 2 Correct Answer Incorrect Answer
    C 3 Correct Answer Incorrect Answer
    D 4 Correct Answer Incorrect Answer

    Solution

    Since, cosx = nsiny
    (nsiny)/(cosy) = m
    ntany = m
    tany = m/n
    tan 2 y = m 2 /n 2
    sec 2 y = 1 + m 2 /n 2  = (m 2  + n 2 )/n 2
    cos 2 y = 1/sec 2 y = n 2 /(m 2  + n 2 )

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