Q. 27. From a point 100 m above a lake, the angle of elevation of a stationary helicopter is 30° and the angle of depression of reflection of thehelicopter in the lake is 60°. Find the height of the helicopter.
Sol. 
Let A be the
stationary helicopter
and F be the reflection
of the helicopter, CB be
the level of lake and D
be the point 100 m
above it
Let AB = height of
helicopter = x m
then BF = x m
BE = DC = 100 m
Let DE = BC = y m
AE = AB EB = (x 100) m
and EF = BF + EB = (x + 100) m
Or
The angle of elevation of an aeroplane from a point on the ground is 60°. After a flight of 15 seconds, the angle of elevation changes to 30°. If the aeroplane is flying at a constant height of 1500
m, find the speed of the aeroplane.
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