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Circles

Question
CBSEENMA10008316

The angle of elevation of the top Q of a vertical tower PQ from a point X on the ground is 60°. At a point Y, 40 m vertically above X, the angle of elevation is 45°. Find the height of the tower PQ and the distance XQ.

Solution
Let PQ be the tower and Y is a point vertically above X such that XY = 40 m. The angle of elevation of the top Q from a point X on the ground is 60° and from a point Y vertically above the X be 45° i.e., ∠PXQ = 60° and ∠RYQ = 45°.

Let    QR = x m
In right triangle QRY, we have
tan space 45 degree space equals space QR over YR
rightwards double arrow space space space 1 space equals space straight x over YR
rightwards double arrow space space space YR space equals space straight x space straight m space space space space space space space space space space space space... left parenthesis straight i right parenthesis
In right triangle QPX, we have
tan space 60 degree space space equals space QP over XP
rightwards double arrow space space square root of 3 space equals space fraction numerator QR space plus PR over denominator XP end fraction
rightwards double arrow space space square root of 3 space equals space fraction numerator straight x plus 40 over denominator XP end fraction space space space space space space left square bracket XY equals PR right square bracket
rightwards double arrow space space XP space equals space fraction numerator straight x plus 40 over denominator square root of 3 end fraction
rightwards double arrow space space YR space equals space fraction numerator straight x plus 40 over denominator square root of 3 end fraction space space space left square bracket because space XP space equals space YR right square bracket space space space space.... left parenthesis ii right parenthesis space
Comparing (i) and (ii), we get
straight x space equals space fraction numerator straight x plus 40 over denominator square root of 3 end fraction
rightwards double arrow space space square root of 3 straight x end root equals space straight x plus 40
rightwards double arrow space space square root of 3 straight x end root minus straight x equals 40
rightwards double arrow space space straight x left parenthesis square root of 3 minus 1 right parenthesis end root space equals space 40
rightwards double arrow space space space straight x space equals fraction numerator 40 over denominator square root of 3 minus 1 end fraction straight x fraction numerator square root of 3 plus 1 over denominator square root of 3 plus 1 end fraction equals fraction numerator 40 left parenthesis square root of 3 plus 1 end root right parenthesis over denominator open parentheses square root of 3 close parentheses squared minus left parenthesis 1 right parenthesis squared end fraction
equals fraction numerator 40 open parentheses square root of 3 plus 1 close parentheses over denominator 2 end fraction equals 20 open parentheses square root of 3 plus 1 close parentheses equals 20 left parenthesis 1.732 plus 1 right parenthesis
equals space 20 space straight x space 2.732 space equals space 54.64

So, height of the tower PQ
= PR + QR
= 40 + x
= 40 + 54.64
= 94.64 m.
In right triangle QPX, we have
sin space 60 degree space equals space PQ over XQ
rightwards double arrow space space space fraction numerator square root of 3 over denominator 2 end fraction equals fraction numerator 94.64 over denominator XQ end fraction
rightwards double arrow space space space XQ space equals space fraction numerator 94.64 space straight X space 2 over denominator square root of 3 end fraction
rightwards double arrow space XQ space equals space fraction numerator 94.64 straight x 2 straight x square root of 3 over denominator 3 end fraction
rightwards double arrow space XQ space equals space 109.3 space straight m

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