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Q.

The pilot of helicopter at an altitude of 1000 m sees two aeroplanes, one on his left and the other on his right at the same height and finds their angles of depression as 45°   and 60°  . What will be the distance between the two aeroplanes?


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a

1577.4m

b

1487.4m

c

1578.4m

d

1577m 

answer is A.

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Detailed Solution

Given that,
The angle of depression of the left aeroplane will be θ= 45 °    The angle of depression of the right aeroplane will be ϕ= 60 °    The altitude of the helicopter from the ground will be h=1000  m  We draw a diagram.
Question ImageIn ΔPRS  ,
PS   the distance from the helicopter to the right aeroplane let x.
SR= the altitude at which the helicopter stays from the ground
From ΔPRS   using tanϕ= height base  ,
tanϕ= SR PS tanϕ= h x x= h tanϕ  
Substituting h  = 1000  m and ϕ= 60 °   in x= h tanϕ   ,
x= 1000 tan 60 ° x= 1000 3 [tan 60 ° = 3 ] x=577.35m  
In ΔSQR  ,
QS = the distance from the helicopter to the left aeroplane let y.
From ΔSQR  ,
  tanθ= SR QS tanθ= h y y= h tanθ  
Substituting h= 1000  m and θ  = 45 °   in y= h tanθ   ,
y= 1000 tan 45 ° y= 1000 1 [tan 45 ° =1] y=1000m  
Using x  = 577.35  m and y= 1000  m and d   is the distance between the aeroplanes,
d=x+y d=1000+577.35 d=1577.4m  
Thus the distance between the aeroplanes will be 1577.4m.
Therefore the correct option is 1.
 
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