An airplane is in level flight at an altitude of 1.5 km from the ground is observed at an angle of elevation of 60°. After 10 seconds, its angle of elevation is observed to be 30°. The speed of the airplane in km/hr is:
Show answer & explanation
Correct answer: A
Explanation
Let the height of the airplane be 1.5 km.
In the first position,
tan60° = 1.5 / x
x = 1.5/√3 = 0.5√3 km
In the second position,
tan30° = 1.5 / y
y = 1.5√3 km
Distance covered in 10 seconds = y − x
= 1.5√3 − 0.5√3
= √3 km
Speed = (√3/10) × 3600 = 360√3 km/h