From an aeroplane vertically above a straight horizontal road ,the angles of depression of two consecutive milestones on

opposite sides of the aeroplane are observed to be A and B. show that height in miles of aeroplane above the road is

tanA tanB / tanA+tanB

please help me

let the plane be at point A at the particular time.

let the height of the plane from the ground be AB= h miles.

C and D be the two consecutive mile stones therefore CD=1 mile, 

such that ∠ACB= A and ∠ADB=B.

now in the right angled triangle ABC,

in the right angled triangle ABD,

 

adding (1) and (2),

which is the required result.

hope this helps you

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