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A bird is sitting on the top of a vertical pole 20 m high and its elevation from a point O on the ground is 45 ° . It flies off horizontally straight away from the point O . After one second, the elevation of the bird from O is reduced to 30 ° . Then the speed (in m / s ) of the bird is

Options

  1. A2 0 2
  2. B2 0 3 - 1
  3. C4 0 2 - 1
  4. D4 0 3 - 2

Correct answer

B. 2 0 3 - 1

Step-by-step solution

​ A D = 2 0   m A O = 2 0 tan 4 5 ∘ = 2 0 1 = 2 0   m C B = 2 0   m ∴ B O = 20 tan 30 ∘ = 20 1 3 = 20 3 m ∴     A B = B O - A O = 2 0 3 - 2 0 = 2 0 3 - 1 m It took 1   sec to travel distance D C . ∴ Speed = 2 0 3 - 1 m / sec .

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