Q53 of 68 Page 9

The angle of elevation of an airplane from a point A on the ground is 60° after a flight of 30 seconds, the angle of elevation changes to 30°. If the plane is flying at a constant height of 3600 m, find the speed, in km/hour, of the plane.


Let us suppose that DE = x and CD = y


Now, In right ΔBED, we have




⇒ x = 3600


In right ΔACE, we have




⇒ CE = 10800


⇒ CD + DE = 10800


⇒ y + x = 10800


⇒ y + 3600 = 10800


⇒ y = 10800 – 3600


⇒ y = 7200


∵ AB = CD = 7200


We know that,





Hence, the speed of the aeroplane is 864 km/hr


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