TOPICS
Exercise - 9.1

Question-1 :-  A circus artist is climbing a 20 m long rope, which is tightly stretched and tied from the top of a vertical pole to the ground. Find the height of the pole, if the angle made by the rope with the ground level is 30° (see Figure). trigonometry

Solution :-
trigonometry
  Here, AB is the Pole.
  Length of rope (AC) = 20 m
  Height of the pole (AB) = ?
  
  In Δ ABC,
  AB/AC = sin 30°
  AB/20 = 1/2
     AB = 20/2
     AB = 10 m
  Therefor, Height of the pole is 10 m.
    

Question-2 :-  A tree breaks due to storm and the broken part bends so that the top of the tree touches the ground making an angle 30° with it. The distance between the foot of the tree to the point where the top touches the ground is 8 m. Find the height of the tree.

Solution :-
trigonometry
  Let AC was a original tree.
  Length of Break part of tree (A'C) = 8 m
  Total Height of the tree (AC) = ?
  
  In Δ A'BC,
  BC/A'C = tan 30°
  BC/8 = 1/√3
    BC = 8/√3 m

  A'C/A'B = cos 30°
    8/A'B = √3/2
      A'B = 16/√3 m
   
  Total Heght of tree (AC) = A'B + BC = 16/√3 + 8/√3 = 24/√3 = 8√3 m
  Therefor, Height of the tree is 8√3 m.
    

Question-3 :-  A contractor plans to install two slides for the children to play in a park. For the children below the age of 5 years, she prefers to have a slide whose top is at a height of 1.5 m, and is inclined at an angle of 30° to the ground, whereas for elder children, she wants to have a steep slide at a height of 3m, and inclined at an angle of 60° to the ground. What should be the length of the slide in each case?

Solution :-
  Let AC and PR be the slides of Younger and Elder Children respectively.
  AB = 1.5 m, PQ = 3 m 
trigonometry
  In Δ ABC,
  AB/AC = sin 30°
  1.5/AC = 1/2
      AC = 3.0 m
  Therefore, length of younger slides is 3 m.
trigonometry
  In Δ PQR,
  PQ/PR = sin 60°
   3/PR = √3/2
     PR = 6/√3 = 2√3 m
  Therefore, length of elder slides is 2√3 m.
    

Question-4 :-  The angle of elevation of the top of a tower from a point on the ground, which is 30 m away from the foot of the tower, is 30°. Find the height of the tower.

Solution :-
trigonometry
  Let AB the tower and Distance BC = 30 m 
  Angle of elevation = 30°
  Height of the tower (AB) = ?

  In Δ ABC,
  AB/BC = tan 30°
  AB/30 = 1/√3
     AB = 30/√3 = 10√3 m
  Therefore, the height of tower is 10√3 m.
    

Question-5 :-  A kite is flying at a height of 60 m above the ground. The string attached to the kite is temporarily tied to a point on the ground. The inclination of the string with the ground is 60°. Find the length of the string, assuming that there is no slack in the string.

Solution :-
trigonometry
  Let K is a Kite.
  Height of kite from the ground (LK) = 60m
  Length of string (KP) = ?

  In Δ KLP,
  KL/KP = sin 60°
  60/KP = √3/2
     KP = 120/√3 = 40√3 m
  Therefore, the Length of string is 40√3 m.
    

Question-6 :-  A 1.5 m tall boy is standing at some distance from a 30 m tall building. The angle of elevation from his eyes to the top of the building increases from 30° to 60° as he walks towards the building. Find the distance he walked towards the building.

Solution :-
trigonometry
  Let a boy was standing at point S initially. 
  He walked towards the building and reached at T point. 
  Height of the Boy = 1.5 m
  Height of the building (PQ) = 30 m
  So, PR = PQ - RQ = 30 - 1.5 = 28.5 = 57/2 m
  Distance from walked towards the building (ST) = ?
  
  In Δ PAR,
  PR/AR = tan 30°
  57/2AR = 1/√3
      AR = (57√3)/2 m

  In Δ PRB,
  PR/BR = tan 60°
  57/2BR = √3
      BR = 57/(2√3) = (19√3)/2 m

  Now, ST = AB 
          = AR - BR 
          = (57√3)/2 - (19√3)/2
          = (38√3)/2
          = 19√3 m
  Therefore, the Distance from walked towards the building is 19√3 m.
    

Question-7 :-  From a point on the ground, the angles of elevation of the bottom and the top of a transmission tower fixed at the top of a 20 m high building are 45° and 60° respectively. Find the height of the tower.

Solution :-
trigonometry
  Let AB be the Transmission Tower.
  Height of building (BC) = 20 m
  AC = AB + BC
  Height of Transmission Tower (AB) = ?

  In Δ BCD,
  BC/CD = tan 45°
  20/CD = 1
     CD = 20 m

  In Δ ACD,
  AC/CD = tan 60°
  (AB + BC)/BC = √3
       AB + BC = √3 BC
    √3 BC - BC = AB
   BC (√3 - 1) = AB
    20(√3 - 1) = AB
            AB = 20(√3 - 1) m 
  Therefore, the height of Transmission tower is 20(√3 - 1) m.  
    

Question-8 :-  A statue, 1.6 m tall, stands on the top of a pedestal. From a point on the ground, the angle of elevation of the top of the statue is 60° and from the same point the angle of elevation of the top of the pedestal is 45°. Find the height of the pedestal.

Solution :-
trigonometry
  Let the height of pedestal is BC.
  Height of statue (AB) = 1.6 m
  AC = AB + BC
  Height of pedestal (BC) = ?

  In Δ BCD,
  BC/CD = tan 45°
  BC/CD = 1
     BC = CD

  In Δ ACD,
  AC/CD = tan 60°
  (AB + BC)/BC = √3
      1.6 + BC = √3 BC
    √3 BC - BC = 1.6
   BC (√3 - 1) = 1.6
            BC = 1.6/(√3 - 1)  By rationalising of Denominator
            BC = 1.6(√3 + 1)/[(√3 - 1)(√3 + 1)]
            BC = 1.6(√3 + 1)/2
            BC = 0.8(√3 + 1) m
  Therefore, the height of the pedestal is 0.8(√3 + 1) m.
    

Question-9 :-  The angle of elevation of the top of a building from the foot of the tower is 30° and the angle of elevation of the top of the tower from the foot of the building is 60°. If the tower is 50 m high, find the height of the building.

Solution :-
trigonometry
  Let AB be a Building.
  Height of tower (CD) = 50 m
  Height of the Building (AB) = ?

  In Δ CDB,
  CD/BD = tan 60°
  50/BD = √3
     BD = 50/√3 m

  In Δ ABD,
  AB/BD = tan 30°
  AB/BD = 1/√3
     AB = BD/√3
     AB = 50/(√3 x √3)
     AB = 50/3 m

  Therefore, the height of the building is 50/3 m.
    

Question-10 :-  Two poles of equal heights are standing opposite each other on either side of the road, which is 80 m wide. From a point between them on the road, the angles of elevation of the top of the poles are 60° and 30°, respectively. Find the height of the poles and the distances of the point from the poles.

Solution :-
trigonometry 
  Let AB and CD are be the two equal poles and BO and Do distances of the point from the poles.
  Distance between both poles (BD) = 80 m
  Height of poles = ?
  Distances of the point from the poles = ?
 
  In Δ ABO,
  AB/BO = tan 60°
  AB/BO = √3
     BO = AB/√3 

  In Δ CDO,
  CD/DO = tan 30°
  CD/DO = 1/√3
  CD/(80 - BO) = 1/√3
         CD √3 = 80 - BO
         CD √3 = 80 - AB/√3
  CD √3 + AB/√3 = 80
  By equal Poles,
  CD = AB
  CD √3 + CD/√3 = 80
  CD(√3 + 1/√3) = 80
       CD(4/√3) = 80
             CD = (80√3)/4
             CD = 20√3 m
          or AB = 20√3 m
  BO = AB/√3 = (20√3)√3 = 20 m
  DO = BD - BO = 80 - 20 = 60 m
  Therefore, the height of Poles are 20√3 m and the point is 20 m and 60 m far from these poles.

    

Question-11 :-  A TV tower stands vertically on a bank of a canal. From a point on the other bank directly opposite the tower, the angle of elevation of the top of the tower is 60°. From another point 20 m away from this point on the line joing this point to the foot of the tower, the angle of elevation of the top of the tower is 30° (see Figure). Find the height of the tower and the width of the canal. trigonometry

Solution :-
trigonometry
  Let AB be a TV Tower and BC be a Canal.
  The height of the tower (AB) and the width of the canal (BC) = ?

  In Δ ABC,
  AB/BC = tan 60°
  AB/BC = √3
     BC = AB/√3 

  In Δ ABD,
  AB/BD = tan 30°
  AB/(BC + CD) = 1/√3
  AB/(AB/√3 + 20) = 1/√3
  (√3 AB)/(AB + 20√3) = 1/√3 
  3AB = AB + 20√3
  3AB - AB = 20√3
       2AB = 20√3
        AB = (20√3)/2
        AB = 10√3 m

  BC = AB/√3 = (10√3)/√3 = 10 m
  Therefore, the height of tower is 10√3 m and width of canal is 10 m.
    

Question-12 :-  From the top of a 7 m high building, the angle of elevation of the top of a cable tower is 60° and the angle of depression of its foot is 45°. Determine the height of the tower.

Solution :-
trigonometry
  Let AB be a building and CD be a cable tower.
  Height of building (AB) = 7 m
  Height of tower = ?

  In Δ ABD,
  AB/BD = tan 45°
  7/BD = 1
    BD = 7 m

  In Δ ACE,
  AE = BD = 7m
  CE/AE = tan 60°
   CE/7 = √3
     CE = 7√3 m
  
  CD = CE + ED = 7√3 + 7 = 7(√3 + 1) m   
  Therefore, the height of cable tower is 7(√3 + 1) m.
  Therefore, the height of tower is 10√3 m.
    

Question-13 :-  As observed from the top of a 75 m high lighthouse from the sea-level, the angles of depression of two ships are 30° and 45°. If one ship is exactly behind the other on the same side of the lighthouse, find the distance between the two ships.

Solution :-
trigonometry
  Let AB be a light house and C and D are be the two ships.
  Height of light house (AB) = 75 m
  The distance between the two ships (CD) = ?

  In Δ ABC,
  AB/BC = tan 45°
  75/BC = 1
     BC = 75 m

  In Δ ABD,
  AB/BD = tan 30°
  75/(BC + CD) = 1/√3
  BC + CD = 75√3 
  75 + CD = 75√3
       CD = 75√3 - 75
       CD = 75(√3 - 1) m

  Therefore, The distance between the two ships 75(√3 - 1) m.
    

Question-14 :-  A 1.2 m tall girl spots a balloon moving with the wind in a horizontal line at a height of 88.2 m from the ground. The angle of elevation of the balloon from the eyes of the girl at any instant is 60°. After some time, the angle of elevation reduces to 30° (see Figure). Find the distance travelled by the balloon during the interval. trigonometry

Solution :-
trigonometry
  Let A and B are be the initial point and final point of baloon from the ground.
  Height of baloon from the ground (BH) or (AF) = 88.2 m  
  Height of girl (CD) = 1.2m
  Distance travelled by the baloon (EG) = ?

  In Δ ACE,
  AE/CE = tan 60°
  (AF - EF)/CE = √3
  (88.2 - 1.2)/CE = √3
            87/CE = √3
               CE = 87/√3 = 29√3 m
     
  In Δ BCG,
  BG/CG = tan 30°
  (BH - GH)/CG = 1/√3
  (88.2 - 1.2)/CG = 1/√3
            87/CG = 1/√3
               CG = 87√3 m

  Therefore, Distance travelled by baloon (EG) = CG - CE = 87√3 - 29√3 = 58√3 m
    

Question-15 :-  A straight highway leads to the foot of a tower. A man standing at the top of the tower observes a car at an angle of depression of 30°, which is approaching the foot of the tower with a uniform speed. Six seconds later, the angle of depression of the car is found to be 60°. Find the time taken by the car to reach the foot of the tower from this point.

Solution :-
trigonometry
  Let AB be a tower and C and D are be the initial and final positions of car.
  Time taken = ?

  In Δ ABD,
  AB/BD = tan 60°
  AB/BD = √3
     BD = AB/√3 

  In Δ ABC,
  AB/BC = tan 30°
  AB/(BD + CD) = 1/√3
     AB √3 = BD + CD
     AB √3 = AB/√3 + CD
        CD = AB √3 - AB/√3
        CD = 2AB/√3

  Therefore, Time taken by the car to travel distance CD (2AB/√3) = 6 seconds
  Time taken by the car to travel distance DB (AB/√3) = 6/(2AB/√3) x (AB/√3) = 6/2 = 3 seconds
    

Question-16 :-  The angles of elevation of the top of a tower from two points at a distance of 4 m and 9 m from the base of the tower and in the same straight line with it are complementary. Prove that the height of the tower is 6 m.

Solution :-
trigonometry
  Let AQ be a tower and Base RQ and SQ are be the 4m and 9m respectively.
  The angles are complementary. If one angle is θ and another is (90° - θ)

  In Δ AQR,
  AQ/QR = tan θ
  AQ/4 = tan θ  .....(1)
  
  In Δ AQS,
  AQ/QS = tan(90 - θ)
  AQ/9 = cot θ  .....(2)
  
  Multipying eq (1) and (2)
  AQ/4 x AQ/9 = tan θ . cot θ
  AQ/36 = tan θ . 1/tan θ
   AQ²/36 = 1
     AQ² = 36
     AQ = √36
     AQ = 6 
  Therefore, the height of tower is 6 m (Hence Proved).
    
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