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      Question

      The angle of elevation of an aeroplane from a point on

      the ground is 60°. After 20 seconds flight the elevation changes to 30°, if the aeroplane is flying at a height of 3000√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 300 m/sec Correct Answer Incorrect Answer

      Solution

      Distance between two place in which angle change is D = n(cotθ₁ + cotθ₂) Distance = 3000√3 (cot30° + cot60°) = 3000√3 × √3 - 1/√3 = 3000√3 × 2/√3 = 6000 m So, distance covered by plane in 20 sec = 6000 m Speed of plane = 6000/20 = 300 m/sec.

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