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Basic Trigonometric Functions

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Right Triangle Trigonometry

Trigonometry is the study of triangles. If you know the angles of a triangle and one side length, you can use the properties of similar triangles and proportions to completely solve for the missing sides.

Imagine trying to measure the height of a flag pole. It would be very difficult to measure vertically because it could be several stories tall. Instead walk 10 feet away and notice that the flag pole makes a 65 degree angle with your feet. Using this information, what is the height of the flag pole? 

Watch This

http://www.youtube.com/watch?v=Ujyl_zQw2zE James Sousa: Introduction to Trigonometric Functions Using Triangles

Guidance

The six trigonometric functions are sine, cosine, tangent, cotangent, secant and cosecant. Opp stands for the side opposite of the angle \theta , hyp stands for hypotenuse and adj stands for side adjacent to the angle \theta

\sin \theta = \frac{opp}{hyp}

\cos \theta = \frac{adj}{hyp}

\tan \theta = \frac{opp}{adj}

\cot \theta =\frac{adj}{opp}

\sec \theta  = \frac{hyp}{adj}

\csc \theta = \frac{hyp}{opp}

The reason why these trigonometric functions exist is because two triangles with the same interior angles will have side lengths that are always proportional. Trigonometric functions are used by identifying two known pieces of information on a triangle and one unknown, setting up and solving for the unknown. Calculators are important because the operations of sin, cos and tan are already programmed in. The other three (cot, sec and csc) are not usually in calculators because there is a reciprocal relationship between them and tan, cos and sec.

\sin \theta = \frac{opp}{hyp} = \frac{1}{\csc \theta}

\cos \theta = \frac{adj}{hyp} = \frac{1}{\sec \theta}

\tan \theta = \frac{opp}{adj} = \frac{1}{\cot \theta}

Keep in mind that your calculator can be in degree mode or radian mode. Be sure you can toggle back and forth so that you are always in the appropriate units for each problem. 

Note: The images throughout this concept are not drawn to scale.

Example A

Solve for side b .

Solution:

\sin \left ( \frac{2 \pi}{7} \right ) & = \frac{b}{14}\\b & = 14 \cdot \sin \left ( \frac{2\pi}{7} \right ) \approx 10.9 \ in

Example B

Solve for angle A .

Solution: This problem can be solved using sin, cos or tan because the opposite, adjacent and hypotenuse lengths are all given. 

The argument of a sin function is always an angle. The arcsin or \sin^{-1} \theta  function on the calculator on the other hand has an argument that is a side ratio. It is useful for finding angles that have that side ratio. 

\sin A & = \frac{5}{13}\\A& = \sin^{-1}\left ( \frac{5}{13} \right ) \approx 0.39 \ radian \approx 22.6^\circ

Example C

Given a right triangle with  a = 12 \ inm\angle B=20^\circ,  and m\angle C=90^\circ , find the length of the hypotenuse. 

Solution:

\cos 20^\circ & = \frac{12}{c}\\c & = \frac{12}{\cos 20^\circ} \approx 12.77 \ in

Concept Problem Revisited

Instead walk 10 feet away and notice that the flag pole makes a 65^\circ  angle with your feet. 

If you walk 10 feet from the base of a flagpole and assume that the flagpole makes a 90^\circ  angle with the ground. 

\tan 65^\circ & = \frac{x}{10}\\x & = 10 \tan 65^\circ \approx 30.8 \ ft

Vocabulary

The six trigonometric ratios are universal proportions that are always true of similar triangles (triangles with congruent corresponding angles).

\theta  (theta) is a Greek letter and is just a letter used in math to stand for an unknown angle. 

Guided Practice

1. Given \triangle ABC where  B is a right angle, m\angle C=18^\circ , and c=12 . What is a ?

2. Given \triangle XYZ where  Z is a right angle, m\angle X=1 \ radian , and x=3 . What is z ?

3. Given \triangle MNO where  O is a right angle,  m=12 , and n=14 . What is the measure of angle M

Answers:

1. Drawing out this triangle, it looks like:

\tan 18^\circ & = \frac{12}{a}\\a & = \frac{12}{\tan 18^\circ} \approx 36.9

2. Drawing out the triangle, it looks like:

\sin 1&=\frac{3}{z}\\z&=\frac{3}{\sin 1} \approx 3.6

3. Drawing out the triangle, it looks like:

\tan M & = \frac{12}{14}\\M&=\tan^{-1} \left ( \frac{12}{14} \right ) \approx 0.7 \ radian \approx 40.6^\circ

Practice

For 1-15, information about the sides and/or angles of right triangle  ABC is given. Completely solve the triangle (find all missing sides and angles) to 1 decimal place.

Problem Number

A

B

C

a

b

c

1.

90^\circ

 

 

 

4

7

2.

90^\circ

 

37^\circ

18

 

 

3.

 

90^\circ

15^\circ

 

32

 

4. 

 

 

90^\circ

6

 

11

5.

90^\circ

12^\circ

 

19

 

 

6.

 

90^\circ

 

 

17

10

7.

90^\circ

10^\circ

 

 

2

 

8.

4^\circ

90^\circ

 

0.3

 

 

9.

\frac{\pi}{2}  radian

 

1 radian

 

 

15

10.

 

\frac{\pi}{2}  radian

 

12

15

 

11.

 

 

\frac{\pi}{2}  radian

 

9

14

12.

\frac{\pi}{4}  radian

\frac{\pi}{4}  radian

 

 

5

 

13.

\frac{\pi}{2}  radian

 

 

26

13

 

14.

 

\frac{\pi}{2}  radian

 

 

19

16

15.

 

 

\frac{\pi}{2}  radian

10

 

10\sqrt{2}

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