An aeroplane flying horizontally at a height of 2500m above the ground is observed at a certain point on earth to subtend and angle of 60 ^{0} . After 15 seconds, its angle of elevation at the same point is observed to be 30 ^{0} . Calculate the speed of the aeroplane.

In ∆ABC,

In ∆AED,

From figure,

BD = CE

and, CE = AE - AC

so, BD = AE - AC

now, substitute value of AE and AC from equation (1) and (2), we get,

BD is covered by aeroplane in 15 seconds. So,

Hence, speed of the aeroplane is 192.455 m/s

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