Two stations due south of a leaning tower which leans towards the north are at distances a and b from its foot. If, α, β, are the elevations of the top of the tower from these stations,
prove that its inclination ө to the horizontal is given by cot  straight theta space equals space fraction numerator straight b space cot space straight phi space minus space straight a space cot space straight beta over denominator straight b space minus straight a space end fraction

Let CE be the leaning tower. Let A and B be two given stations at distances a and b respectively from the foot of the tower.
Let CD = x and DE = h
In right triangle CDE, we have

tan space straight theta space equals space DE over CD
rightwards double arrow space space tan space straight theta space space equals space straight h over straight x

Let CE be the leaning tower. Let A and B be two given stations at dis
rightwards double arrow space space straight x space equals space fraction numerator straight h over denominator tan space straight theta end fraction equals straight h space cot space straight theta space space space space space space space space space space space space space space space space space... left parenthesis straight i right parenthesis
In right triangle BDE, we have

tan space straight alpha space space equals space DE over BD
rightwards double arrow space space tan space straight alpha space equals space fraction numerator DE over denominator BC plus CD end fraction
rightwards double arrow space space tan space straight alpha space equals space space fraction numerator straight h over denominator straight a plus straight x end fraction
rightwards double arrow space space straight a space plus space straight x space equals space fraction numerator straight h over denominator tan space straight alpha end fraction
rightwards double arrow space space space straight a space plus space straight x space equals space straight h space cot space straight alpha
rightwards double arrow space space space space straight x space equals space straight h space cot space straight alpha space minus space straight a 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 ADE, we have

tan space straight beta space equals space DE over AD
rightwards double arrow space tan space straight beta space equals space fraction numerator DE over denominator AC plus CD end fraction
rightwards double arrow space space tan space straight beta space equals space fraction numerator straight h over denominator straight b plus straight x end fraction
rightwards double arrow space space straight b plus straight x equals fraction numerator straight h over denominator tan space straight beta end fraction
rightwards double arrow space space straight b plus straight x equals space straight h space cot space straight beta
rightwards double arrow space space straight x space equals space straight h space cot space straight beta space minus space straight b space space space space space space space space space space space... left parenthesis iii right parenthesis
Comparing (i) and (ii), we get

straight h space cot space straight theta space equals space straight h space cot space straight alpha space minus space straight a
rightwards double arrow space straight h space cot space straight alpha space minus space straight h space cot space straight theta space equals space straight a
rightwards double arrow space straight h space left parenthesis cot space straight alpha space minus space cot space straight theta right parenthesis space equals space straight a
straight h space equals space fraction numerator straight a over denominator cot space straight alpha space minus space cot space straight theta end fraction space space space space space space space space space space space space space space space space space space space space space... left parenthesis iv right parenthesis

Comparing (i) and (iii), we get

straight h space cot space straight theta space equals space straight h space cot space straight beta space minus space straight b
rightwards double arrow space straight h space cot space straight beta space minus space straight h space cot space straight theta space equals space straight b
rightwards double arrow space straight h space left parenthesis cot space straight beta space minus space cot space straight theta right parenthesis space equals space straight b
straight h space equals space fraction numerator straight b over denominator cot space straight beta space minus space cot space straight theta end fraction space space space space space space space space space space space space space space space space space space... left parenthesis straight v right parenthesis

Comparing (iv) and (v), we get

fraction numerator straight a over denominator cot space straight alpha space minus space cot space straight theta end fraction space equals space fraction numerator straight b over denominator cot space straight beta minus cot space straight theta end fraction

rightwards double arrow space space straight a left parenthesis cot space straight beta space minus space cot space straight theta right parenthesis space equals space straight b left parenthesis cot space straight alpha space minus space cot space straight theta right parenthesis
rightwards double arrow space space straight a left parenthesis cot space straight beta space minus space straight a space cot space straight theta right parenthesis space equals space straight b space cot space straight alpha space minus space straight b space cot space straight theta
rightwards double arrow space space straight b space cot space straight theta space minus straight a space cot space straight theta space equals straight b space cot space straight alpha space minus space straight a space cot space straight beta
rightwards double arrow space cot space straight theta space left parenthesis straight b minus straight a right parenthesis space equals space straight b space cot space straight alpha minus space straight a space cot space straight beta
rightwards double arrow space space space space space space space cot space straight theta space equals space fraction numerator straight b space cot space straight alpha space minus space straight a space cot space straight beta over denominator straight b minus straight a end fraction
Hence, inclination ө to the horizontal is given by cot straight theta space equals space fraction numerator straight b space cot space straight alpha space minus space straight a space cot space straight beta over denominator straight b minus straight a end fraction


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A boy is standing on the ground and flying a kite with a string of length 150 m, at an angle of elevation of 30°. Another boy is standing on the roof of a 25 m high building and is flying his kite at an elevation of 45°. Both the boys are on opposite sides of both the kites. Find the length of the string (in mts), correct to two decimal places, that the second boy must have so that the two kites meet.

Let A be the boy and F is the position of kite. Let BF be vertical height of the kite and AF is the length of the string i.e. AF = 150 m. It is given that ∠BAF = 30°.


Let A be the boy and F is the position of kite. Let BF be vertical he
In right triangle ABF, we have

sin space 30 degree space space equals space BF over AF
rightwards double arrow space space 1 half equals fraction numerator BE plus EF over denominator AF end fraction
rightwards double arrow space space 1 half equals fraction numerator 25 plus EF over denominator 150 end fraction
rightwards double arrow space space space 2 left parenthesis 25 plus EF right parenthesis space equals space 150
rightwards double arrow space space space 50 space plus space 2 EF space equals space 150
rightwards double arrow space space space space space space 2 EF space space space equals space 100
rightwards double arrow space space space space space space space space EF space space equals space 50 space space space space space space

Let D be the position of second boy and DF be the length of second kite. It is given ∠EDF = 45°.
In right triangle DEF, we have

sin space 45 degree space equals space EF over FD
rightwards double arrow space space fraction numerator 1 over denominator square root of 2 end fraction equals 50 over FD
rightwards double arrow space space space FD space space equals space 50 square root of 2
Hence, the length of the string = 50 square root of 2 space straight m.

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The angle of elevation of a cliff from a fixed point is ө. After going up a distance of K metres towards the top of the cliff at an angle of φ, it is found that the angle of elevation
is α. Show that the height of the cliff is  fraction numerator straight k left parenthesis cos space straight phi space minus space sin space straight phi. space cot space straight alpha right parenthesis over denominator cot space straight theta space minus space cot space straight alpha end fraction.

Let CE be a cliff of height h m. Angle of elevation of cliff from a fixed point A be ө.


Let CE be a cliff of height h m. Angle of elevation of cliff from a f

i.e., ∠CAE = ө. ∠CAF = φ and AF = k metres. From F draw FD and FB perpendiculars on CE and AC respectively. It is also given that ∠DFE = α.
In right triangle ABF, we have

cos space straight phi space equals space AB over AF
rightwards double arrow space space space space cos space straight phi space equals space AB over straight K
rightwards double arrow space space space space AB space equals space straight K space cos space straight phi
and space space sin space straight phi space space equals space BF over AF
rightwards double arrow space space space space sin space straight phi space BF over straight K
rightwards double arrow space space space space space BF space equals space straight k space sin space straight phi

In right triangle ACE, we have

tan space straight theta space equals space CE over AC
rightwards double arrow space space tan space straight theta space equals space straight h over AC
rightwards double arrow space space AC space equals space fraction numerator straight h over denominator tan space straight theta end fraction space equals space straight h space cot space straight theta
Now comma space space space space DF space equals space BC space equals space AC minus AB
space space space space space space space space space space space space space equals space straight h space cot space straight theta space minus space straight k space cos space straight phi

And,    DE = CE - CD = CE - BF
= h - k sin φ.
In right triangle DFE, we have

tan space straight alpha space space equals space DE over DF
rightwards double arrow space space space space fraction numerator 1 over denominator cot space straight alpha end fraction equals fraction numerator straight h space minus space straight K space sin space straight phi over denominator straight h space cot space straight theta space minus space straight K space cos space straight phi end fraction

rightwards double arrow space straight h space cot space straight theta space minus straight K space cos space straight phi space equals space cot space straight alpha space left parenthesis straight h minus straight K space sin space straight phi right parenthesis
rightwards double arrow space straight h space cot space straight theta space minus space straight K space cos space straight phi space equals space space straight h space cot space straight alpha space minus space straight K space sin space straight phi space cot space straight alpha
rightwards double arrow space straight h space cot space straight theta space minus space straight h space cot space straight alpha space equals space space straight k space cos space straight phi space minus space straight k space sin space straight phi space cot space straight alpha
rightwards double arrow space straight h space equals space fraction numerator straight K left parenthesis cos space straight phi space minus space sin space straight phi space cot space straight alpha right parenthesis over denominator cot space straight theta space minus space cot space straight alpha end fraction

Hence, the height of the cliff is

fraction numerator straight K open parentheses cos space straight phi space minus space sin space straight phi. space cot space straight alpha close parentheses over denominator cot space straight theta space minus space cot space straight alpha end fraction

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Two ships are sailing in the sea on the either side of the lighthouse, the angles of depression of two ships as observed from the top of the lighthouse are 60° and 45° respectively. If the distance between the ships is 200 open parentheses fraction numerator square root of 3 plus 1 over denominator square root of 3 end fraction close parentheses straight m. Find the height of the lighthouse.

Let AD be the lighthouse of height h m B and C are the position of the two ships which are on the either side of the road. The angles of depression of the ships from the top of the light house be 60° and 45°, ie., ∠ABD = 60 and ∠ACD = 45°


Let AD be the lighthouse of height h m B and C are the position of th

Let BD = x m and CD = y m.
In right triangle ABD, we have

tan space 60 degree space equals space AD over BD
rightwards double arrow space space space square root of 3 equals straight h over straight x
rightwards double arrow space space space space straight x space equals space fraction numerator straight h over denominator square root of 3 end fraction space space space space space space space space space space space... left parenthesis straight i right parenthesis
In right triangle ACD, we have

tan space 45 degree space space equals space AD over CD
rightwards double arrow space space space space space space space space space space space space space space 1 space equals space straight h over straight y
rightwards double arrow space space space space space space space space space space space space space space space straight y space equals space straight h space space space space space space space space space space space space space space... left parenthesis ii right parenthesis
Adding (i) and (ii), we get

straight x space plus space straight y space equals space fraction numerator straight h over denominator square root of 3 end fraction space plus space straight h
rightwards double arrow space space 200 open parentheses fraction numerator square root of 3 plus 1 over denominator square root of 3 end fraction close parentheses equals fraction numerator straight h plus square root of 3 straight h end root over denominator square root of 3 end fraction
rightwards double arrow space 200 space open parentheses fraction numerator square root of 3 plus 1 over denominator square root of 3 end fraction close parentheses equals fraction numerator straight h left parenthesis 1 plus square root of 3 right parenthesis over denominator square root of 3 end fraction
rightwards double arrow space space space space straight h space equals space 200
Hence, the height of the light house = 200 m.

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A pole 5 m high is fixed on the top of a tower. The angle of elevation of the top of the pole observed from a point A on the ground is 60° and the angle of depression of the point A from the top of the tower is 45°. Find the height of the tower.


Let BC be the height of the tower and CD be the pole of height 5m fixed on the top of the tower. Let BC = h m.


Let BC be the height of the tower and CD be the pole of height 5m fix

The angle of elevation of top of the pole from point A on the ground be 60° and the angle of depression of the point A from the top of the tower be 45°, i.e. ∠ BAD = 60° and ∠ BAC = 45°.
In right triangle ABC, we have

tan space 45 degree space equals space BC over AB
rightwards double arrow space space space 1 space space equals space straight h over AB
rightwards double arrow space space space AB space equals space space straight h space space space space space space space space space space space space space space space space space space space space space space... left parenthesis straight i right parenthesis space space space space space space space space space space space space space space space space space space space space space space

In right triangle ABD, we have

tan space 60 degree space equals space space BD over AB
rightwards double arrow space space square root of 3 equals fraction numerator BC plus CD over denominator AB end fraction
rightwards double arrow space space square root of 3 equals fraction numerator straight h plus 5 over denominator AB end fraction
rightwards double arrow space space AB space equals space fraction numerator straight h plus 5 over denominator square root of 3 end fraction 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
Comparing (i) and (ii), we get

rightwards double arrow space space space space space space space space square root of 3 space straight h space minus space straight h space equals space 5
rightwards double arrow space space space space space space space space straight h left parenthesis square root of 3 space minus space 1 right parenthesis space equals space 5
rightwards double arrow space space space space space space straight h space equals space fraction numerator 5 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
rightwards double arrow space space space space space space straight h space equals space fraction numerator 5 left parenthesis square root of 3 plus 1 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 5 left parenthesis square root of 3 plus 1 right parenthesis over denominator 2 end fraction
straight h space equals space fraction numerator straight h plus 5 over denominator square root of 3 equals end fraction
rightwards double arrow space space space square root of 3 straight h space equals space straight h plus 5 space space space space space equals space fraction numerator 5 straight x 2.732 over denominator 2 end fraction equals 5 straight x 1.366 space equals space 3.83 space straight m
Hence, height of the tower be 6.83 m.




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