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Q.

The angle of elevation of the top of a vertical tower from a point on the ground is 60 °  . From another point 10 m   vertically above the first, its angle of elevation is 30 °  . Find the height of the tower.

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a

5 m

b

10 m

c

15 m

d

20 m 

answer is C.

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Detailed Solution

Given that the angle of elevation of the top of a vertical tower from a point on the ground is 60 °   . From another point 10 m vertically above the first, its angle of elevation is 30 °   .
We have to find the height of the tower.
In a right-angle triangle, tanθ=  Opposite side   Hypotenuse   
Question ImageLet the height of the tower be h cm.   Now, In ΔPAB   tan 60 ° = AP AB 3 = h AB AB= h 3 (1)   And, In ΔPCD   tan 30 °  =  PD CD 1 3 = h-10 CD CD= 3 (h-10)----(2)   Since, AB=CD,  
So, equation (2) becomes,
AB =  3 (h-10)----(3)   Equating equation (1) and (3), we get,
h 3  =  3 (h-10) h = 3(h-10) 2 h = 30 h = 15 m   Hence, the height of the tower will be 15 m.
Therefore, the correct option is 3.
 
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