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      Question

      A boat is being rowed away from a cliff 270 m high. From

      the top of the cliff, the angle of depression of the boat changes from 60Β° to 45Β° in 2 minutes. What is the speed of the boat?
      A 1.8 km/h Correct Answer Incorrect Answer
      B 3.4 km/h Correct Answer Incorrect Answer
      C 2.03 km/h Correct Answer Incorrect Answer
      D 4.5 km/h Correct Answer Incorrect Answer

      Solution

      According to the figure above: tan 45Β° = 270/(a + b) β‡’ 1 = 270/(a + b) β‡’ a + b = 270 Also, tan 60Β° = 270/a β‡’ √ 3 = 270/a β‡’ a = 270/ √ 3 β‡’ a = 90√ 3 = 155.8 ∴ b = 270 - 155.8 = 114.2 β‡’ The boat covers a distance of 114.2 m in 2 minutes. ∴ Speed of the boat = (114.2 x 60)/(2 x 1000) km/h = 3.4 km/h

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