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    Question

    If cos 39° = x/ √(x2 + y2), 

     then the value of tan 51° = ?
    A y/x Correct Answer Incorrect Answer
    B 1 Correct Answer Incorrect Answer
    C x/y Correct Answer Incorrect Answer
    D x/√(1+x²) Correct Answer Incorrect Answer

    Solution

    cos 39° = x/ √(x 2 + y 2) =   Base/hypotenuse So base = x &  hypotenuse =  so perpendicular = y But for angle 51° which is complimentary of 39° , perpendicular =y & base = x. So tan 51° = base/perpendicular  =  Alternate Method : cos 39° =  OR  sin (90°-39°)  =  OR sin 51° =     -----(1) Squaring on both sides, we will get, sin2 51° =  or 1 - cos2 51° =  or cos2 51° = 1 -  =  So cos 51° =   ----(2) Hence tan 51° = sin51°/ cos51° =

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