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Question
Mon December 12, 2011 By:

# An aeroplane flying horizontal 1 Km above the ground is observed at an elevation of 60. After 10 seconds its elevation is observed to be 30. Find speed of the plane.

Tue December 13, 2011
When the elevation was 60, distance of the point of observation to the point on the ground
below the plane = 1xcot60 = 1/?3 km

Similarly when the angle becomes 30, distance becomes = 1xcot30 = ?3 km

So, distance travelled = ?3 -1/?3 km = 2/?3 km

time taken = 10sec = 10/3600 hr

hence speed = distance / time = 2x3600/10x?3 = 720/?3 km/hr = 415.7 km/hr
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