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Questions  

 If the angles of elevations of the top of a tower from three collinear points A,B and C on a 

 line leading to the foot of the tower are 30,45 , and 60 respectively, then the ratio AB:BC is 

a
3:2
b
1:3
c
2:3
d
3:1

detailed solution

Correct option is D

Let ' h ' be the height of the tower MN From ΔMCN,tan⁡60=hx⇒x=h cot60°,and from ∆MBN,tan 45=hBN⇒h=BN⇒ BC=h−x⇒BC=h-hcot60° From ΔMAN,tan⁡30=hAB+h⇒AB+h=h cot30°⇒ AB=hcot⁡30∘−h∴ABBC=hcot⁡30∘−1h1−cot⁡60∘=3−11−13=3∴AB:BC=3:1

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The angle of elevation of top of an unfinished tower at a point distance 120m from its base is 450. How much height must the tower be raise so that angle of elevation of top at the same point to be 600.


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