Q. 214.3( 3 Votes )

On the level ground, the angle of elevation of a tower is 30°. On moving 20 m nearer, the angle of elevation is 60°. The height of the tower is
A. 10 m

B. 10√3 m

C. 15 m

D. 20 m

Answer :

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Let AB be the tower and C, D are two points on the ground. Join B, C, D. Also, join A with C and D. We get two right-angled triangles ABC and ABD with right angle at B. The angles of elevation of the top of the tower from the points D and C are 30° and 60° respectively. So, ADB = 30° and ACB = 30°. We are to find the height of the tower. Also given that the distance from D to C is 20 m. So CD = 20 m.


In ∆ABC,



or,



In ∆ABD,



or,



or,



or,



So, the correct option is (B).

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