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Circles

Question
CBSEENMA10008340

At a point on level ground, the angle of elevation of a vertical tower is found to be such that its tangent is 5/12. On walking 192 metres towards the tower, the tangents of the angle is found to be 3/4. Find the height of the tower.

Solution
Let AB be the tower of height lim.D and C are points on the level ground such that distance between them are 192 mts. Let the angles of elevation of a vertical tower from points D and C be α and β respectively.

i.e.,    ∠ACB = β
and    ∠ADB = α
Let    BC = x metres
In right triangle ABC, we have
tan space straight beta space equals space AB over BC
rightwards double arrow space space space tan space straight beta space equals space straight h over straight x
rightwards double arrow space space space 3 over 4 space equals space straight h over straight x
rightwards double arrow space space space space 3 straight x space equals space 4 straight h
rightwards double arrow space space space space straight x space equals space fraction numerator 4 straight h over denominator 3 end fraction space space space space space space space space space space space space space space space space space space space space space space space space space space... left parenthesis ii right parenthesis
In right triangle ABD, we have
tan space straight alpha space equals space AB over BD
rightwards double arrow space space tan space straight alpha space equals space fraction numerator straight h over denominator straight x plus 192 end fraction
rightwards double arrow space space 5 left parenthesis straight x plus 192 right parenthesis space equals space 12 straight h
rightwards double arrow space space 5 straight x space plus space 960 space equals space 12 straight h
rightwards double arrow space space 5 straight x space equals space 12 straight h space minus space 960
rightwards double arrow space space straight x space equals space fraction numerator 12 straight h minus 960 over denominator 5 end fraction space space space space space space space space space space space space... left parenthesis ii right parenthesis
Comparing (i) and (ii), we get
fraction numerator 4 straight h over denominator 3 end fraction equals fraction numerator 12 straight h minus 960 over denominator 5 end fraction
rightwards double arrow space space 20 straight h space equals space 3 space left parenthesis 12 straight h space minus space 960 right parenthesis
rightwards double arrow space space 20 straight h space equals space 36 straight h space equals space minus 2880
rightwards double arrow space space minus 16 straight h space equals space minus 2880
rightwards double arrow space space space space space space straight h space equals space 180 space straight m
Hence, the height of the tower be 180 mts.

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