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An aeroplane flying horizontally 1km above the ground is observed at an elevation of 60 degree.After 10 seconds, its elevation is observed to be 30 degree.

Find the speed of the aeroplane in km/hr.



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Tan60°= 1/d =√3
⇒d =1/√3 km 
now, the aeroplane cover (d+x) km
tan30°= 1/(d+x)=1/√3
⇒(d+x) = √3
⇒x = √3 -(1/√3) = (3-1)/√3 =2/√3= (2×1.732)/3 = 1.15 km
so speed =distance/time = 1.15/(10/3600) =414 km/hr
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