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Q.

Two pillars of equal height stand on either side of the roadway which is 30 m wide. At a point in the roadway between the pillars, the elevations of the tops of the pillars are 60 °  and 30 °  . What will be the heights of the pillars and the position of the point?


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a

15 3 2 m  , 7.5m

b

15 3 4 m  , 7.0m

c

17 3 2 m  , 7.2m

d

17 3 4 m  , 6.9m 

answer is A.

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Detailed Solution

Given that,
The width of the road = 30  m
The angles made by towers = 60 °  , and 30 °  
We draw the diagram,
Question ImageThe height of the equal pillars AB and CD = y
The width of the road BD=30  m
BE=x   then, DE=30x  
Using the identity tanθ= Perpendicular Base   in ΔABE  ,
tanθ= AB BE   Substituting BE=x, AB= y, and θ  = 60 °   in the equation tanθ= AB BE   ,
tan 60 ° = y x 3 = y x [tan 60 ° = 3 ] x= y 3  
Using the identity tanθ= Perpendicular Base   in ΔCDE   ,
  tanθ= CD DE   Substituting DE= 30x   , CD=y and θ  = 30 °   in the equation tanθ= CD DE  ,
tan 30 ° = y 30x 1 3 = y 30x [tan 30 ° = 1 3 ] 30x= 3 y  
The value of y,
Substituting  x  = y 3    in the equation 30x= 3 y  ,
30 y 3 = 3 y 30= 3y+y 3 y= 30 3 4 y= 15 3 2 m   Thus, the height of the equal pillars will be 15 3 2 m.  
Substituting y= 15 3 2 m   in equation x= y 3   ,
x= 15 3 2 3 x=7.5m  
Thus the heights of both pillars will be 15 3 2 m   , and the position of the point will be 7.5  m.
Therefore the correct option is 1.
 
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