Introduction
One of the most important applications of trigonometry is the application of measurement of heights and distances which cannot be measured directly.
It is also extensively used by astronomers in determining the distance of the heavenly bodies like the sun and moon and stars.
Two angles are very often used in the practical applications of trigonometry .
Line of sight
The line of object is the line from our eyes to object.
Angle of Elevation
If the object is above the horizontal level of our eyes, we have to turn our head upwards to view the object. This is called the angle of elevation.
Angle of Depression
If the object is below the horizontal level of our eyes, we have to bend our head to view it. This is called the angle of elevation.
Example 1: The angle of elevation of the top of a tower at a distance of 100 metres from its foot on a horizontal plane is found to be 60^{o}. Find the height of the tower.
Let CA be the tower equal to h metres in length and B point at a distance of 100 metres from its foot C. It is given that angle ABC = 60^{o}
From the right angle triangle ABC we have
h/100 = tan 60
Solving the above we get h = 173.2 metres
Answer: Height of the tower is 173.2 metres
Example 2: From the top of a cliff, 200 metres high, the angle of depression of the top and bottom of a tower are observed to be observed to be 30^{o} and 60^{o}, find the height of the tower.
Solution: Let AB represent the tower and P the top of cliff MP. If PX be the horizontal line through P, then angle XPA = 30^{o} and angle XPB = 60^{o}. Let the height of the tower be h metres. From A draw AL perpendicular to PM.
ML = AB = h=> LP = (200h)
Again, ang PBM = ang XPB = 60
ang PAL = ang XPA = 30 (alternate angles )
From the right triangle PMB, BM/200 = cot 60
BM = 200 cot 60 = 200/3
From the right triangle PLA, AL /AP = cot 30 =>AL = LP cot 30 = =Ö(200h)3
But AL = BM = =Ö(200h)3 =200/3
Answer: h = 133 1/3 metres
Directions: Answer the following questions. Also write at least 10 examples of your own.
