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Special Angles and their Trig Functions By Jeannie Taylor Through Funding Provided by a VCCS LearningWare Grant

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Special Angles and their Trig Functions

By Jeannie Taylor

Through Funding Provided by a

VCCS LearningWare Grant

The quadrantal angles are those angles that lie on the axis of the Cartesian coordinate system: , , , and .

We will first look at the special angles called the quadrantal angles.

If we look at half of that angle, we have

Looking at the angle half-way between 0 and 180 or , we have 90 or .

Looking at the angle half-way between 180 and 360 , we have 270 or radians which is of the total (360 or ).

Moving all the way around from 0 to 360 completes the circle and and gives the 360 angle which is equal to radians.

radians

We also need to be able to recognize these angles when they are given to us in radian measure. Look at the smallest possible positive angle in standard position, other than 0 , yet having the same terminal side as 0 . This is a 360 angle which is equivalent to .

We can count the quadrantal angles in terms of .

Notice that after counting these angles based on portions of the full circle, two of these angles reduce to radians with which we are familiar, .

0 radians

radians

We can approximate the radian measure of each angle to two decimal places. One of them, you already know, . It will probably be a good idea to memorize the others. Knowing all of these numbers allows you to quickly identify the location of any angle.

Add the equivalent degree measure to each of these quadrantal angles.

We can find the trigonometric functions of the quadrantal angles using this definition. We will begin with the point (1, 0) on the x axis.

(1, 0)

0 radians

radians

or

For the angle 0 , we can see that x = 1 and y = 0. To visualize the length of r, think about the line of a 1 angle getting closer and closer to 0 at the point (1, 0).

Remember the six trigonometric functions defined using a point (x, y) on the terminal side of an angle, .

As this line falls on top of the x axis, we can see that the length of r is 1.

Using the values, x = 1, y = 0, and r = 1, we list the six trig functions of 0. And of course, these values also apply to 0 radians, 360 , 2 radians, etc.

(1, 0)

0 radians

radians

or

It will be just as easy to find the trig functions of the remaining quadrantal angles using the point (x, y) and the r value of 1.

The first angle, half way between 0 and would be .

0

Since 0 to radians is 90 , we know that is half of 90 or 45. Each successive angle is 45 more than the previous angle. Now we can name all of these special angles in degrees.

We can again count around the circle, but this time we will count in terms of radians. Counting we say:

It is much easier to construct this picture of angles in both degrees and radians than it is to memorize a table involving these angles (45 or reference angles,).

Now let’s cut each quadrant in half, which basically gives us 8 equal sections.

Then reduce appropriately.

Next we will look at two special triangles: the 45 – 45 – 90 triangle and the 30 – 60 – 90 triangle. These triangles will allow us to easily find the trig functions of the special angles, 45 , 30 , and 60 .

The lengths of the legs of the 45 – 45 – 90 triangle are equal to each other because their corresponding angles are equal.

1

1

If we let each leg have a length of 1, then we find the hypotenuse to be using the Pythagorean theorem.

45

45

You should memorize this triangle or at least be able to construct it. These angles will be used frequently.

Using the definition of the trigonometric functions as the ratios of the sides of a right triangle, we can now list all six trig functions for a angle.

1

1

45

45

For the 30 – 60 – 90 triangle, we will construct an equilateral triangle (a triangle with 3 equal angles of each, which guarantees 3 equal sides).

If we let each side be a length of 2, then cutting the triangle in half will give us a right triangle with a base of 1 and a hypotenuse of 2. This smaller triangle now has angles of 30, 60, and 90 .

2

1

We find the length of the other leg to be , using the Pythagorean theorem.

You should memorize this triangle or at least be able to construct it. These angles, also, will be used frequently.

Again, using the definition of the trigonometric functions as the ratios of the sides of a right triangle, we can now list all the trig functions for a 30 angle and a 60 angle.

2

1

1

2

Either memorizing or learning how to construct these triangles is much easier than memorizing tables for the 45 , 30 , and 60 angles. These angles are used frequently and often you need exact function values rather than rounded values. You cannot get exact values on your calculator.

1

45

45

1

2

Sine II

All I

1

45

45

III

Tangent

IV

Cosine

Knowing these triangles, understanding the use of reference angles, and remembering how to get the proper sign of a function enables us to find exact values of these special angles.

A good way to remember this chart is that ASTC stands for All Students Take Calculus.

Example 1: Find the six trig functions of 330 .

First draw the 330 degree angle.

Second, find the reference angle, 360 - 330 = 30

To compute the trig functions of the 30 angle, draw the “special” triangle.

y

2

A

S

1

30

x

330

T

C

Determine the correct sign for the trig functions of 330 . Only the cosine and the secant are “+”.

First determine the location of .

With a denominator of 3, the distance from 0 to radians is cut into thirds. Count around the Cartesian coordinate system beginning at 0 until we get to .

y

We can see that the reference angle is , which is the same as 60 . Therefore, we will compute the trig functions of using the 60 angle of the special triangle.

2

1

x

Example 2: Find the six trig functions of . (Slide 1)

Example 2: Find the six trig functions of . (Slide 2)

Before we write the functions, we need to determine the signs for each function. Remember “All Students Take Calculus”. Since the angle, , is located in the 3rd quadrant, only the tangent and cotangent are positive. All the other functions are negative..

y

2

A

S

1

x

T

C

We see that the angle is located in the 2nd quadrant and the cos is negative in the 2nd quadrant.

1

0 radians

We know that is the same as 45 , so the reference angle is 45 . Using the special triangle we can see that the cos of 45 or is .

A

Note that the reference angle is .

S

45

45

T

C

cos =

Example 3: Find the exact value of cos .

We will first draw the angle to determine the quadrant.

- Find the value of the sec 360 without using a calculator.
- Find the exact value of the tan 420 .
- Find the exact value of sin .
- Find the tan 270 without using a calculator.
- Find the exact value of the csc .
- Find the exact value of the cot (-225 ).
- Find the exact value of the sin .
- Find the exact value of the cos .
- Find the value of the cos(- ) without using a calculator.
- Find the exact value of the sec 315 .

Key For The Practice Exercises

- sec 360 = 1
- tan 420 =
- sin =
- tan 270 is undefined
- csc =
- cot (-225 ) = -1
- sin =
- cos =
- cos(- ) = -1
- sec 315 =

Problems 3 and 7 have solution explanations following this key.

We will first draw the angle by counting in a negative direction in units of .

We can see that is the reference angle and we know that is the same as 30 . So we will draw our 30 triangle and see that the sin 30 is .

2

0 radians

A

S

1

T

C

Answer: sin =

Problem 3: Find the sin .

All that’s left is to find the correct sign.

And we can see that the correct sign is “-”, since the sin is always “-” in the 3rd quadrant.

We see that the angle is located in the 3rd quadrant and the cos is negative in the 3rd quadrant.

1

0 radians

We know that is the same as 45 , so the reference angle is 45 . Using the special triangle we can see that the cos of 45 or is .

A

Note that the reference angle is .

S

45

45

T

C

cos =

Problem 7: Find the exact value of cos .

We will first draw the angle to determine the quadrant.

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