What if you were given a right triangle and told that its sides measure 3, 4, and 5 inches? How could you find the sine, cosine, and tangent of one of the triangle's nonright angles? After completing this Concept, you'll be able to solve for these trigonometric ratios.
Watch This
CK12 Foundation: Chapter8SoneCosineTangentA
Watch the parts of the video dealing with the sine, cosine, and tangent.
James Sousa: Introduction to Trigonometric Functions Using Triangles
Guidance
The word trigonometry comes from two words meaning triangle and measure. In this lesson we will define three trigonometric (or trig) functions.
Trigonometry: The study of the relationships between the sides and angles of right triangles.
In trigonometry, sides are named in reference to a particular 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. We never reference the right angle when referring to trig ratios.
The three basic trig ratios are called, sine, cosine and tangent. At this point, we will only take the sine, cosine and tangent of acute angles. However, you will learn that you can use these ratios with obtuse angles as well.
Sine Ratio: For an acute angle
Cosine Ratio: For an acute angle
Tangent Ratio: For an acute angle
There are a few important things to note about the way we write these ratios. First, keep in mind that the abbreviations
A few important points:
 Always reduce ratios when you can.
 Use the Pythagorean Theorem to find the missing side (if there is one).
 The tangent ratio can be bigger than 1 (the other two cannot).
 If two right triangles are similar, then their sine, cosine, and tangent ratios will be the same (because they will reduce to the same ratio).
 If there is a radical in the denominator, rationalize the denominator.
 The sine, cosine and tangent for an angle are fixed.
Example A
Find the sine, cosine and tangent ratios of
First, we need to use the Pythagorean Theorem to find the length of the hypotenuse.
So,
Example B
Find the sine, cosine, and tangent of
Find the length of the missing side.
Therefore,
Example C
Find the sine, cosine and tangent of
This is a special right triangle, a 306090 triangle. So, if the short leg is 6, then the long leg is
Watch this video for help with the Examples above.
CK12 Foundation: Chapter8SineCosineTangentB
Concept Problem Revisited
The trigonometric ratios for the nonright angles in the triangle above are:
Guided Practice
Answer the questions about the following image. Reduce all fractions.
1. What is
2. What is
3. What is
Answers:
1.
2.
3.
Explore More
Use the diagram to fill in the blanks below.

tanD=?? 
sinF=?? 
tanF=?? 
cosF=?? 
sinD=?? 
cosD=??
From questions 16, we can conclude the following. Fill in the blanks.

cos−−−−=sinF andsin−−−−=cosF . 
tanD andtanF are _________ of each other.
Find the sine, cosine and tangent of
 Explain why the sine of an angle will never be greater than 1.
 Explain why the tangent of a
45∘ angle will always be 1.  As the degree of an angle increases, will the tangent of the angle increase or decrease? Explain.