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    Question

    The angle of elevation of an aeroplane from a point on

    the ground is 60°. After 12 seconds flight the elevation changes to 30°, if the aeroplane is flying at a height of 1200√3 m. find the speed of the aeroplane.
    A 200 m/sec Correct Answer Incorrect Answer
    B 100 m/sec Correct Answer Incorrect Answer
    C 150 m/sec Correct Answer Incorrect Answer
    D 300 m/sec Correct Answer Incorrect Answer
    E None of these Correct Answer Incorrect Answer

    Solution

    Distance between two place in which angle change is D = n(cotθ₁ + cotθ₂) Distance = 1200√3 (cot30° + cot60°) = 1200√3 × √3 - 1/√3 = 1200√3 ×2/√3 = 2400 m So, distance covered by plane in 12 sec = 2400 m Speed of plane = 2400/12 = 200 m/sec.

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