Question
The angle of elevation of a jet plane from a point A on the ground is 60°. After a flight of 15 seconds the angle of elevation changes to 30°. If the jet plane is flying at a constant height of 
find the speed of the jet plane.
Solution
Let A be the point of observation, C and E be the two points of the plane. It is given that after 15 seconds angle of elevation changes from 60° to 30°.
i.e., ∠BAC = 60° and ∠DAE = 30°. It is also given that height of the jet plane is 1500
[Since jet plane is flying at constant height, therefore, CB = ED =
In right triangle ABC, we have
In right triangle ADE, we have
i.e., ∠BAC = 60° and ∠DAE = 30°. It is also given that height of the jet plane is 1500
[Since jet plane is flying at constant height, therefore, CB = ED =
In right triangle ABC, we have
In right triangle ADE, we have
Putting the value of (i) in (ii), we get
1500 + BD = 4500
⇒ BD = 3000
∵ Distance travelled in 15 sec
= CE = BD = 3000 metres,
Now, speed of plane (m/s) = 
and speed of plane (km/h) = 
= 720 km/hr
