5.2: Identifying Trigonometric Ratios
Learning Objectives
 Identify the different parts of right triangles.
 Identify complementary angles in right triangles.
 Become familiar with the basic trigonometric ratios of sine, cosine, and tangent.
What is Trigonometry?
Trigonometry is the study of triangles and the relationships between their side lengths and the angles in between their sides.
The word Trigonometry has two parts: “trig” means triangle and “metry” means measure
Where is Trigonometry used?
There are many applications of trigonometry. Of particular value is the technique of triangulation, which is used in astronomy to measure the distance to nearby stars, in geography to measure distances between landmarks, and in satellite navigation systems. GPSs (global positioning systems) would not be possible without trigonometry. Other fields which make use of trigonometry include:
 astronomy (and navigation  on the oceans, in aircraft, and in space)
 music theory and acoustics
 analysis of financial markets
 electronics
 probability theory and statistics
 medical imaging (CAT scans and ultrasound)
 pharmacy, chemistry, and biology
 number theory (and cryptology)
 land surveying and geodesy
 architecture
 various types of engineering (electrical, mechanical, and civil)
 computer graphics
 cartography (the study of maps)
In your own words, Trigonometry is:
What are the three most interesting applications of trigonometry for you?
Parts of a Triangle
In trigonometry, there are a number of different labels attributed to different sides of a right triangle. They are usually in relation to a specific angle. The hypotenuse of a triangle is always the same, but the terms adjacent and opposite depend on which angle you are referencing.
A side adjacent to an angle is the leg of the triangle that helps form the angle.
A side opposite to an angle is the leg of the triangle that does not help form the angle.
What does the word adjacent mean?
Adjacent means “next to,” so an adjacent side is next to the angle in question.
Examine the picture below: Segment
In the triangle shown above, segment
Similarly,
The hypotenuse is always
In the picture above,
Segment
Segment
Because angle
Example 1
Examine the triangle in the diagram below.
Identify which leg is adjacent to
The first part of the question asks you to identify the leg adjacent to
The next part of the question asks you to identify the leg opposite
Since an opposite leg is the leg that does not help to form the angle, it must be
The hypotenuse is always opposite the right angle, so in this triangle the hypotenuse is segment
Complementary Angles in Right Triangles
Recall that in all triangles, the sum of the measures of all angles must be
Reading Check:
1. What is the sum of all three angles in a triangle?
2. What is the sum of any two complementary angles?
3. What is the study of triangles called?
Example 2
What is the measure of
To find
So, the measure of
If angle
Trigonometric Ratios
The fundamentals of trigonometry are the trigonometric functions. There are three basic trigonometric functions: sine, cosine and tangent. These are abbreviated to: sin, cos, and tan:
These functions are defined from a rightangled triangle.
Consider a rightangled triangle:
In trigonometry, it is very common to see the letter
You can think of
In the rightangled triangle, we refer to the lengths of the three sides according to how they are placed in relation to the angle
The Greek letter _____ is used to represent an angle in trigonometry.
The side opposite to
We define:
These functions relate the lengths of the sides of a triangle to its interior angles.
One way to remember the definitions is to use the first letter of each word. Notice the bold letters in the definitions above spell out:
SOHCAHTOA
This stands for the trigonometric functions and which sides correspond to each of them:
Sine is Opposite over Hypotenuse (SOH)
Cosine is Adjacent over Hypotenuse (CAH)
Tan is Opposite over Adjacent (TOA)
IMPORTANT: The definitions of opposite, adjacent and hypotenuse only make sense when you are working with rightangled triangles! Always check to make sure your triangle has a rightangle before you use them; otherwise you will get the wrong answer.
Graphic Organizer: SOHCAHTOA
Trigonometric Function  Trig Function Abbreviation  Letters stand for:  Ratio of Side Lengths  Draw a picture  Write yourself some notes to help you remember! 

Sine 
S O H 


Cosine 
C A H 


Tangent 
T O A 

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