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    Question

    The angle of elevation of an aeroplane from a point on

    the ground is 60°. After 15 seconds flight the elevation changes to 30°, if the aeroplane is flying at a height of 2400√3 m. find the speed of the aeroplane.
    A 240 m/sec Correct Answer Incorrect Answer
    B 120 m/sec Correct Answer Incorrect Answer
    C 160 m/sec Correct Answer Incorrect Answer
    D 320 m/sec Correct Answer Incorrect Answer

    Solution

    Distance between two place in which angle change is  D = n(cotθ₁ + cotθ₂) Distance = 2400√3 (cot30° + cot60°) = 2400√3 × √3 - 1/√3  = 2400√3 × 2/√3 = 4800 m So, distance covered by plane in 15 sec = 4800 m Speed of plane = 4800/15 = 320 m/sec.

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