photo taken by Joe Sueyoshi
A student will be able to:
In the previous lesson we introduced the six trigonometric functions, and we worked with these functions in two ways: first, in right triangles, and second, for angles of rotation. In this lesson we will extend our work with trig functions of angles of rotation to any angle in the unit circle, including negative angles, and angles greater than . In the previous lesson, we worked with the quadrantal angles, and with the angles , , and . In this lesson we will work with angles related to these angles, as well as other angles in the unit circle. One of the key ideas of this lesson is that angles may share the same trig values. This idea will be developed throughout the lesson.
In the previous lesson, one of the review questions asked you to consider the angle . If we graph this angle in standard position, we see that the terminal side of this angle is a reflection of the terminal side of , across the axis.
Notice that makes a angle with the negative axis. Therefore we say that is the reference angle for . Formally, the reference angle of an angle in standard position is the angle formed with the closest portion of the axis. Notice that is the reference angle for many angles. For example, it is the reference angle for and for .
Review Video: Determining trig function values using refence triangles
Review Video: Determining trig function values using the unit circle
Unit Circle: You Try It Animation
In general, identifying the reference angle for an angle will help you determine the values of the trig functions of the angle.
Example 1: Graph each angle and identify its reference angle.
a. makes a angle with the axis. Therefore the reference angle is .
b. makes a with the axis. Therefore the reference angle is .
c. is a full rotation of , plus an additional . So this angle is co-terminal with , and is its reference angle.
If an angle has a reference angle of , , or , we can identify its ordered pair on the unit circle, and so we can find the values of the six trig functions of that angle. For example, above we stated that has a reference angle of . Because of its relationship to , the ordered pair for is is . Now we can find the values of the six trig functions of :
Example 2: Find the ordered pair for and use it to find the value of .
As we found in example 1, the reference angle for is . The figure below shows and the three other angles in the unit circle that have as a reference angle.
The terminal side of the angle represents a reflection of the terminal side of over both axes. So the coordinates of the point are . The coordinate is the sine value, so .
Just as the figure above shows and three related angles, we can make similar graphs for and .
Knowing these ordered pairs will help you find the value of any of the trig functions for these angles.
Example 3: Find the value of
Using the graph above, you will find that the ordered pair is . Therefore the cotangent value is
We can also use the concept of a reference angle and the ordered pairs we have identified to determine the values of the trig functions for other angles.
Recall that graphing a negative angle means rotating clockwise. The graph below shows .
Notice that this angle is coterminal with . So the ordered pair is . We can use this ordered pair to find the values of any of the trig functions of . For example, .
In general, if a negative angle has a reference angle of ,, or , or if it is a quadrantal angle, we can find its ordered pair, and so we can determine the values of any of the trig functions of the angle.
Example 4: Find the value of each expression.
is in the quadrant, and has a reference angle of . That is, this angle is coterminal with . Therefore the ordered pair is and the sine value is .
The angle is in the quadrant and has a reference angle of . That is, this angle is coterminal with . Therefore the ordered pair is and the secant value is .
The angle is coterminal with . Therefore the ordered pair is and the cosine value is .
We can also use our knowledge of reference angles and ordered pairs to find the values of trig functions of angles with measure greater than .
Consider the angle . As you learned previously, you can think of this angle as a full degree rotation, plus an additional . Therefore is coterminal with . As you saw above with negative angles, this means that has the same ordered pair as , and so it has the same trig values. For example,
In general, if an angle whose measure is greater than has a reference angle of , , or , or if it is a quadrantal angle, we can find its ordered pair, and so we can find the values of any of the trig functions of the angle. The first step is to determine the reference angle.
Example 5: Find the value of each expression.
is a full rotation of , plus an additional . Therefore the angle is coterminal with , and so it shares the same ordered pair, . The sine value is the coordinate.
is two full rotations, or , plus an additional :
Therefore is coterminal with , so the ordered pair is . The tangent value can be found by the following:
is a full rotation of , plus an additional . Therefore the angle is coterminal with , and the ordered pair is . So the cosine value is .
So far all of the angles we have worked with are multiples of and . Next we will find approximate values of the trig functions of other angles.
As you work through this chapter, you will learn about different applications of the trig functions. In many cases, you will need to find the value of a function of an angle that is not necessarily one of the "special" angles we have worked with so far. Traditionally, textbooks have provided students with tables that contain values of the trig functions. Below is a table that provides approximate values of the sine, cosine, and tangent values of several angles.
We can use the table to identify approximate values.
Example 6: Find the approximate value of each expression, using the table above.
We can identify the sine value by finding the row in the table for . The sine value is found in the third row of the table. Note that this is an approximate value. We can evaluate the reasonableness of this value by thinking about an angle that is close to , . We know that the ordered pair for is , so the sine value is , which is also in the table. It is reasonable that which is slightly less than the sine value of , given where the terminal sides of these angles intersect the unit circle.
We can identify this cosine value by finding the row for . The cosine value is found in the second column. Again, we can determine if this value is reasonable by considering a nearby angle. is between and , and its cosine value is between the cosine values of these two angles.
We can identify this tangent value by finding the row for , and reading the final column of the table. In the review questions, you will be asked to explain why the tangent value seems reasonable.
Video: Determining trig function values on the calculator
If you have a scientific calculator, you can determine the value of any trig function for any angle. Here we will focus on using a TI graphing calculator to find values.
First, your calculator needs to be in the correct "mode." In chapter 2 you will learn about a different system for measuring angles, known as radian measure. In this chapter, we are measuring angles in degrees. (This is analogous to measuring distance in miles or in kilometers. It's just a different system of measurement.) We need to make sure that the calculator is working in degrees. To do this, press [MODE]. You will see that the third row says Radian Degree. If Degree is highlighted, you are in the correct mode. If Radian is highlighted, scroll down to this row, scroll over to Degree, and press [ENTER]. This will highlight Degree. Then press [Mode] to return to the main screen.
Now you can calculate any value. For example, we can verify the values from the table above. To find , press [SIN][ENTER]. The calculator should return the value .
You may have noticed that the calculator provides a "(" after the SIN. In the previous calculation, you can actually leave off the ")". However, in more complicated calculations, leaving off the closing ")" can create problems. It is a good idea to get in the habit of closing parentheses.
You can also use a calculator to find values of more complicated expressions.
Example 7: Use a calculator to find an approximate value of . Round your answer to places.
To use a TI graphing calculator, press [SIN][+][COS][ENTER]. The calculator should return the number . This rounds to
How does the calculator determine trig function values?
Related articleshttp://www.homeschoolmath.net/teaching/sine_calculator.php http://en.wikipedia.org/wiki/Exact_trigonometric_constants
In this lesson we have examined the idea that we can find an exact or an approximate value of each of the six trig functions for any angle. We began by defining the idea of a reference angle, which is useful for finding the ordered pair for certain angles in the unit circle. We have found exact values of the trig functions for "special" angles, including negative angles, and angles whose measures are greater than . We have also found approximations of values for other angles, using a table, and using a calculator. In the coming lessons, we will use the ideas from this lesson to (1) examine relationships among the trig functions and (2) apply trig functions to real situations.
Consider the following functions:
Do you observe any patterns in these functions? Are there any equalities among the functions? Can you make a general conjecture about and for all values of , ?
What about and ?