The angle of elevation of a jet fighter from point A on the ground is 60°.After a flight of 15 seconds, the angle of elevation changes to 30°.If the jet is flying at a speed of 720 km/h, find the constant height at which jet is flying.

let A be the observation point. and 

let B and C be the two positions of jet fighter. it takes 15 sec. to reach the point C from B.

let the height of the jet fighter from the ground be BE = CD = h km

the speed of the jet = 720 km/hr =720000m/hr

distance = speed*time

therefore BC =

in the right angled triangle BEA,

in the right angled triangle CDA,

AD = AE+ED = AE+BC

thus the height of the jet fighter is 2.598 km .

hope this helps you.

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