Learning Objectives
In this section, you will:
- Find function values for the sine and cosine of [latex]30^{\circ}\text{ or }\left(\frac{\pi }{6}\right),\text{ }45^{\circ}\text{ or }\left(\frac{\pi }{4}\right)[/latex] and [latex]{60^{\circ}}\text{or}\left(\frac{\pi }{3}\right).[/latex]
- Identify the domain and range of sine and cosine functions.
- Use reference angles to evaluate trigonometric functions.
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Finding Function Values for the Sine and Cosine
To define our trigonometric functions, we begin by drawing a unit circle, a circle centered at the origin with radius 1, as shown in Figure 2. The angle (in radians) that [latex]t[/latex] intercepts forms an arc of length [latex]s.[/latex] Using the formula [latex]s=rt,[/latex] and knowing that [latex]r=1,[/latex] we see that for a unit circle, [latex]s=t.[/latex]
Recall that the x- and y-axes divide the coordinate plane into four quarters called quadrants. We label these quadrants to mimic the direction a positive angle would sweep. The four quadrants are labeled I, II, III, and IV.
For any angle [latex]t,[/latex] we can label the intersection of the terminal side and the unit circle by its coordinates, [latex]\left(x,y\right).[/latex] Consider an angle that is in the first quadrant. We can drop a perpendicular line to the x-axis to create a right triangle. The sides of the right triangle will be x and y. If we use our right trigonometric definitions from Section 3.1, we can see that [latex]\mathrm{cos}\left(t\right)=\frac{x}{1}[/latex] and [latex]\mathrm{sin}\left(t\right)=\frac{y}{1}.[/latex] This means [latex]\left(x,\text{ }y\right)\text{ }=\text{ }\left(\mathrm{cos}\left(t\right),\text{ }\mathrm{sin}\left(t\right)\right).[/latex]
Definition
A unit circle has a center at [latex]\left(0,0\right)[/latex] and radius [latex]1[/latex] . In a unit circle, the length of the intercepted arc is equal to the radian measure of the central angle [latex]t\text{.}[/latex]
Let [latex]\left(x,y\right)[/latex] be the endpoint on the unit circle of an arc of arc length [latex]t.[/latex] The [latex]\left(x,y\right)[/latex] coordinates of this point can be described as functions of the angle [latex]t[/latex] where:
[latex]\mathrm{cos}\left(t\right)=x[/latex] and
[latex]\mathrm{sin}\left(t\right)=y.[/latex]
Note that this definition allows us to use angles which are not acute or in other words, angles in standard position whose terminal side is not in the first quadrant.
Now that we have our unit circle labeled, we can learn how the [latex]\left(x,y\right)[/latex] coordinates relate to the arc length and angle. The sine function relates a real number [latex]t[/latex] to the y-coordinate of the point where the corresponding angle intercepts the unit circle. More precisely, the sine of an angle [latex]t[/latex] equals the y-value of the endpoint on the unit circle of an arc of length [latex]t.[/latex] In Figure 3, the sine is equal to [latex]y.[/latex] Like all functions, the sine function has an input and an output. Its input is the measure of the angle; its output is the y-coordinate of the corresponding point on the unit circle.
The cosine function of an angle [latex]t[/latex] equals the x-value of the endpoint on the unit circle of an arc of length [latex]t.[/latex] In Figure 3, the cosine is equal to [latex]x.[/latex]
Important Note: Because it is understood that sine and cosine are functions, we do not always need to write them with parentheses: [latex]\mathrm{sin}\text{ }t[/latex] is the same as [latex]\mathrm{sin}\left(t\right)[/latex] and [latex]\mathrm{cos}\text{ }t[/latex] is the same as [latex]\mathrm{cos}\left(t\right).[/latex] Likewise, [latex]{\mathrm{cos}}^{2}\left(t\right)[/latex] is a commonly used shorthand notation for [latex]{\left(\mathrm{cos}\left(t\right)\right)}^{2}.[/latex] Be aware that many calculators and computers do not recognize the shorthand notation. When in doubt, use the extra parentheses when entering calculations into a calculator or computer.
How To
Given a point P [latex]\left(x,y\right)[/latex] on the unit circle corresponding to an angle of [latex]t,[/latex] find the sine and cosine.
- The sine of [latex]t[/latex] is equal to the y-coordinate of point [latex]P:\mathrm{sin}\left(t\right)=y.[/latex]
- The cosine of [latex]t[/latex] is equal to the x-coordinate of point [latex]P: \text{cos}\left(t\right)=x.[/latex]
Example 1: Finding Function Values for Sine and Cosine
Point [latex]P[/latex] is a point on the unit circle corresponding to an angle of [latex]t,[/latex] as shown in Figure 4. Find [latex]\mathrm{cos}\left(t\right)[/latex] and [latex]\mathrm{sin}\left(t\right).[/latex]
Try It #1
A certain angle [latex]t[/latex] corresponds to a point on the unit circle at [latex]\left(-\frac{\sqrt[\leftroot{1}\uproot{2} ]{2}}{2},\frac{\sqrt[\leftroot{1}\uproot{2} ]{2}}{2}\right)[/latex] as shown in Figure 5. Find [latex]\mathrm{cos}\left(t\right)[/latex] and [latex]\mathrm{sin}\left(t\right).[/latex]
Finding Sines and Cosines of Angles on an Axis
For quadrantral angles, the corresponding point on the unit circle falls on the x- or y-axis. In that case, we can easily calculate cosine and sine from the values of [latex]x[/latex] and [latex]y.[/latex]
Example 2: Calculating Sines and Cosines Along an Axis
Find [latex]\mathrm{cos}\left(90^{\circ}\right)[/latex] and [latex]\text{sin}\left(90^{\circ}\right).[/latex]
Try It #2
Find cosine and sine of the angle [latex]\pi .[/latex]
The Pythagorean Identity
Now that we can define sine and cosine, we will learn how they relate to each other and the unit circle. Recall that the equation for the unit circle is [latex]{x}^{2}+{y}^{2}=1.[/latex] Because [latex]x=\mathrm{cos}\left(t\right)[/latex] and [latex]y=\mathrm{sin}\left(t\right),[/latex] we can substitute for [latex]x[/latex] and [latex]y[/latex] to get [latex]\mathrm{cos}^{2}\left(t\right)+\mathrm{sin}^{2}\left(t\right)=1.[/latex] This equation, [latex]\mathrm{cos}^{2}\left(t\right)+\mathrm{sin}^{2}\left(t\right)=1,[/latex] is known as the Pythagorean Identity. See Figure 7.
We can use the Pythagorean Identity to find the cosine of an angle if we know the sine, or vice versa. However, because the equation yields two solutions, we need additional knowledge of the angle to choose the solution with the correct sign. If we know the quadrant where the angle is, we can easily choose the correct solution.
Definition
The Pythagorean Identity states that, for any real number [latex]t,[/latex]
How To
Given the sine of some angle [latex]t[/latex] and its quadrant location, find the cosine of [latex]t.[/latex]
- Substitute the known value of [latex]\mathrm{sin}\left(t\right)[/latex] into the Pythagorean Identity.
- Solve for [latex]\mathrm{cos}\left(t\right).[/latex]
- Choose the solution with the appropriate sign for the x-values in the quadrant where [latex]t[/latex] is located.
Example 3: Finding a Cosine from a Sine or a Sine from a Cosine
If [latex]\mathrm{sin}\left(t\right)=\frac{3}{7}[/latex] and [latex]t[/latex] is in the second quadrant, find [latex]\mathrm{cos}\left(t\right).[/latex]
Try It #3
If [latex]\mathrm{cos}\left(t\right)=\frac{24}{25}[/latex] and [latex]t[/latex] is in the fourth quadrant, find [latex]\text{sin}\left(t\right).[/latex]
Finding Sines and Cosines of Special Angles
We have already learned some properties of the special angles, such as the conversion from radians to degrees. In section 3.1, we also calculated sines and cosines of the special angles using the Pythagorean Identity and our knowledge of triangles.
Finding Sines and Cosines of 45° Angles and 30° and 60° Angles
We have already found the cosine and sine values for all of the most commonly encountered angles in the first quadrant of the unit circle. Figure 9 summarizes these values.
Angle | 0 | [latex]\frac{\pi }{6},[/latex] or 30° | [latex]\frac{\pi }{4},[/latex] or 45° | [latex]\frac{\pi }{3},[/latex] or 60° | [latex]\frac{\pi }{2},[/latex] or 90° |
Cosine | 1 | [latex]\frac{\sqrt[\leftroot{1}\uproot{2} ]{3}}{2}[/latex] | [latex]\frac{\sqrt[\leftroot{1}\uproot{2} ]{2}}{2}[/latex] | [latex]\frac{1}{2}[/latex] | 0 |
Sine | 0 | [latex]\frac{1}{2}[/latex] | [latex]\frac{\sqrt[\leftroot{1}\uproot{2} ]{2}}{2}[/latex] | [latex]\frac{\sqrt[\leftroot{1}\uproot{2} ]{3}}{2}[/latex] | 1 |
Using a Calculator to Find Sine and Cosine
To find the cosine and sine of angles other than the special angles, we turn to a computer or calculator. Be aware: Most calculators can be set into “degree” or “radian” mode, which tells the calculator the units for the input value. When we evaluate [latex]\mathrm{cos}\left(30\right)[/latex] on our calculator, it will evaluate it as the cosine of 30 degrees if the calculator is in degree mode, or the cosine of 30 radians if the calculator is in radian mode.
How To
Given an angle in radians, use a graphing calculator to find the cosine.
- If the calculator has degree mode and radian mode, set it to radian mode.
- Press the COS key.
- Enter the radian value of the angle and press the close-parentheses key “)”.
- Press ENTER.
Example 4: Using a Graphing Calculator to Find Sine and Cosine
Evaluate [latex]\mathrm{cos}\left(5.1\right)[/latex] using a graphing calculator or computer.
Try It #4
Evaluate [latex]\mathrm{sin}\left(2.3\right).[/latex]
Reference Angles
For any given angle in the first quadrant, there is an angle in the second quadrant with the same y-value and therefore the same sine value. Because the sine value is the y-coordinate on the unit circle, the other angle with the same sine will share the same y-value, but have the opposite x-value. Therefore, its cosine value will be the opposite of the first angle’s cosine value.
Likewise, there will be an angle in the fourth quadrant with the same x-value and therefore the same cosine as the original angle in the first quadrant. The angle with the same cosine will share the same x-value but will have the opposite y-value. Therefore, its sine value will be the opposite of the original angle’s sine value.
As shown in Figure 11, angle [latex]\alpha[/latex] has the same sine value as angle [latex]t;[/latex] the cosine values are opposites. Angle [latex]\beta[/latex] has the same cosine value as angle [latex]t;[/latex] the sine values are opposites.
Finding Reference Angles
An angle’s reference angle is the measure of the smallest, positive, acute angle [latex]t[/latex] formed by the terminal side of the angle [latex]t[/latex] and the horizontal axis. Thus positive reference angles have terminal sides that lie in the first quadrant and can be used as models for angles in other quadrants.
Definition
An angle’s reference angle is the size of the smallest acute angle, [latex]{t}^{\prime },[/latex] formed by the terminal side of the angle [latex]t[/latex] and the horizontal axis.
We can see that reference angle is always an angle between [latex]0^{\circ}[/latex] and [latex]90^{\circ},[/latex] or [latex]0[/latex] and [latex]\frac{\pi }{2}[/latex] radians. As we can see from Figure 12, for any angle in quadrants II, III, or IV, there is a reference angle in quadrant I.
How To
Given an angle between [latex]0[/latex] and [latex]2\pi,[/latex] or between [latex]0^{\circ}[/latex] and [latex]360^{\circ},[/latex] find its reference angle.
- An angle in the first quadrant is its own reference angle.
- For an angle in the second quadrant, the reference angle is [latex]\pi-t[/latex] or [latex]180^{\circ}-t.[/latex]
- For an angle in the third quadrant, the reference angle is [latex]t-\pi[/latex] or [latex]t-180^{\circ}.[/latex]
- For an angle in the fourth quadrant, the reference angle is [latex]2\pi -t[/latex] or [latex]360^{\circ}-t.[/latex]
Given an angle less than [latex]0[/latex] or greater than [latex]2\pi,[/latex] or less than [latex]0^{\circ}[/latex] or greater than [latex]360^{\circ},[/latex] find its reference angle.
- If an angle is less than [latex]0[/latex] or greater than [latex]2\pi,[/latex] add or subtract [latex]2\pi[/latex] as many times as needed to find a coterminal angle between [latex]0[/latex] and [latex]2\pi .[/latex] Similarly, if working in degrees add or subtract [latex]360^{\circ}[/latex] as many times as needed to find a coterminal angle between [latex]0^{\circ}[/latex] and [latex]360^{\circ}.[/latex]
- Once you have an angle between [latex]0[/latex] and [latex]2\pi,[/latex] or between [latex]0^{\circ}[/latex] and [latex]360^{\circ},[/latex] follow steps 1 – 4 above.
Example 5: Finding a Reference Angle
Find the reference angle of [latex]225^{\circ}[/latex] as shown in Figure 13.
Try It # 5
Find the reference angle of [latex]\frac{5\pi }{3}.[/latex]
Using Reference Angles to Find Exact Values for Cosine and Sine
We can find the cosine and sine of any angle in any quadrant if we know the cosine or sine of its reference angle. The absolute values of the cosine and sine of an angle are the same as those of the reference angle. The sign depends on the quadrant of the original angle. The cosine will be positive or negative depending on the sign of the x-values in that quadrant. The sine will be positive or negative depending on the sign of the y-values in that quadrant.
For an angle which has a special angle as the reference angle ([latex]\frac{\pi}{6}, \frac{\pi}{4},[/latex] or [latex]\frac{\pi}{3},[/latex]) we can produce exact value outputs for sine and cosine.
How To
Given an angle in standard position, find the reference angle, and the cosine and sine of the original angle.
- Measure the angle between the terminal side of the given angle and the horizontal axis. That is the reference angle.
- Determine the values of the cosine and sine of the reference angle.
- Give the cosine the same sign as the x-values in the quadrant of the original angle.
- Give the sine the same sign as the y-values in the quadrant of the original angle.
Example 6: Using Reference Angles to Find Sine and Cosine
- Using a reference angle, find the exact values of [latex]\mathrm{cos}\left(150^{\circ}\right)[/latex] and [latex]\mathrm{sin}\left(150^{\circ}\right).[/latex]
- Find angles between [latex]0^{\circ}[/latex] and [latex]360^{\circ}[/latex] which have the same exact values as [latex]\mathrm{cos}\left(150^{\circ}\right)[/latex] and [latex]\mathrm{sin}\left(150^{\circ}\right).[/latex]
Try It #6
- Use the reference angle of [latex]315^{\circ}[/latex] to find the exact values for [latex]\mathrm{cos}\left(315^{\circ}\right)[/latex] and [latex]\mathrm{sin}\left(315^{\circ}\right).[/latex]
- Find angles between [latex]0^{\circ}[/latex] and [latex]360^{\circ}[/latex] which have the same exact values as [latex]\mathrm{cos}\left(315°\right)[/latex] and [latex]\mathrm{sin}\left(315^{\circ}\right).[/latex]
Example 7: Using Reference Angles to Find Sine and Cosine
- Using a reference angle, find the exact values of [latex]\mathrm{cos}\left(\frac{5\pi}{4}\right)[/latex] and [latex]\mathrm{sin}\left(\frac{5\pi}{4}\right).[/latex]
- Find angles between [latex]0\text{ and }2\pi[/latex] which have the same exact values as [latex]\mathrm{cos}\left(\frac{5\pi}{4}\right)[/latex] and [latex]\mathrm{sin}\left(\frac{5\pi}{4}\right).[/latex]
Try It # 7
- Use the reference angle of [latex]-\frac{\pi }{6}[/latex] to find the exact values of [latex]\mathrm{cos}\left(-\frac{\pi}{6}\right)[/latex] and [latex]\mathrm{sin}\left(-\frac{\pi}{6}\right).[/latex]
- Find angles between [latex]0\text{ and }2\pi[/latex] which have the same exact values as [latex]\mathrm{cos}\left(-\frac{\pi}{6}\right)[/latex] and [latex]\mathrm{sin}\left(-\frac{\pi}{6}\right).[/latex]
Using Reference Angles to Find Coordinates
Now that we have learned how to find the cosine and sine values for angles whose reference angles are acute special angles, the rest of the special angles on the unit circle can be determine. They are shown in Figure 14. Take time to learn the [latex]\left(x,y\right)[/latex] coordinates of all of the major angles in the first quadrant.
In addition to learning the values for special angles, we can use reference angles to find [latex]\left(x,y\right)[/latex] coordinates of any point on the unit circle, using what we know of reference angles along with the identities
First we find the reference angle corresponding to the given angle. Then we take the sine and cosine values of the reference angle, and give them the signs corresponding to the y– and x-values of the quadrant.
How To
Given the angle of a point on a unit circle, find the [latex]\left(x,y\right)[/latex] coordinates of the point.
- Find the reference angle by measuring the smallest angle to the x-axis.
- Find the cosine and sine of the reference angle.
- Determine the appropriate signs for [latex]x[/latex] and [latex]y[/latex] in the given quadrant.
Example 8: Using the Unit Circle to Find Coordinates
Find the coordinates of the point on the unit circle at an angle of [latex]\frac{7\pi }{6}.[/latex]
Try It # 8
Find the coordinates of the point on the unit circle at an angle of [latex]\frac{5\pi }{3}.[/latex]
Circles with Radius Different Than 1
Suppose we have a circle centered at the origin other than the unit circle. How can we find the coordinates of a point where the terminal side of the angle in standard position intersects the circle? We know from Section 3.1 that the sine and cosine values of an acute angle do not change regardless of the size of the triangle. This means that [latex]\mathrm{cos}\left(t\right)[/latex] will be the same whether [latex]t[/latex] is in a triangle with small or large sides. However, it is clear that the coordinates of the point of intersection of the terminal side with the circle of radius r will not be the same as the coordinates of the point on the unit circle.
We can apply similar reasoning to considering values of sine and cosine values on a circle that is not a unit circle as we did earlier in this section. By drawing an angle in standard position in quadrant 1, we can drop a perpendicular from the point of intersection of the terminal side with the circle whose center is at the origin to the x axis. If the point on the circle is (x, y), then the sides of the right triangle formed by dropping the perpendicular will also be [latex]x\text{ and }y.[/latex] Now, instead of the radius being 1, we would have a radius of [latex]r.[/latex] Therefore, [latex]\mathrm{cos}\left(t\right)=\frac{x}{r}\text{ and }\mathrm{sin}\left(t\right)=\frac{y}{r}.[/latex] This means that [latex]x=r\mathrm{cos}\left(t\right)\text{ and }y=r\mathrm{sin}\left(t\right).[/latex]
Definition
A point [latex]\left(x,y\right)[/latex] on the circle, centered at the origin, with radius [latex]r[/latex] corresponds to
Example 9: Finding Coordinates on a Circle with Radius [latex]r[/latex]
Find the coordinates of the point on the circle with radius 5 at an angle of [latex]\frac{7\pi}{6}.[/latex]
Try It # 9
Find the coordinates of the point on the circle with radius 7 at an angle of [latex]\frac{5\pi }{3}.[/latex]
Even and Odd Functions
To be able to use our sine and cosine functions freely with both positive and negative angle inputs, we should examine how each function treats a negative input. As it turns out, there is an important difference among the functions in this regard.
Recall that:
- An even function is one in which [latex]f\left(-x\right)=f\left(x\right).[/latex]
- An odd function is one in which [latex]f\left(-x\right)=-f\left(x\right).[/latex]
We can test whether a trigonometric function is even or odd by drawing a unit circle with a positive and a negative angle, as in Figure 15. The sine of the positive angle is [latex]y.[/latex] The sine of the negative angle is [latex]-y.[/latex]The sine function, then, is an odd function. The cosine of the positive angle is [latex]x[/latex], as is the cosine of the negative angle. Therefore, the cosine function is an even function.
We can summarize this by saying:
[latex]\begin{align*}\mathrm{sin}\left(-x\right)&=-\mathrm{sin}\left(x\right)\\ \mathrm{cos}\left(-x\right)&=\mathrm{cos}\left(x\right)\end{align*}[/latex]
Access these online resources for additional instruction and practice with sine and cosine functions.
Key Equations
Cosine | [latex]\mathrm{cos}\left(t\right)=x[/latex] |
Sine | [latex]\mathrm{sin}\left(t\right)=y[/latex] |
Pythagorean Identity | [latex]{\mathrm{cos}}^{2}\left(t\right)+{\mathrm{sin}}^{2}\left(t\right)=1[/latex] |
Key Concepts
- Finding the function values for the sine and cosine begins with drawing a unit circle, which is centered at the origin and has a radius of 1 unit.
- Using the unit circle, the sine of an angle [latex]t[/latex] equals the y-value of the endpoint on the unit circle of an arc of length [latex]t[/latex] whereas the cosine of an angle [latex]t[/latex] equals the x-value of the endpoint.
- When the sine or cosine is known, we can use the Pythagorean Identity to find the other. The Pythagorean Identity is also useful for determining the sines and cosines of special angles.
- Calculators and graphing software are helpful for finding sines and cosines if the proper procedure for entering information is known.
- The domain of the sine and cosine functions is all real numbers.
- The range of both the sine and cosine functions is [latex]\left[-1,1\right].[/latex]
- The sine and cosine of an angle have the same absolute value as the sine and cosine of its reference angle.
- The signs of the sine and cosine are determined from the x– and y-values in the quadrant of the original angle.
- An angle’s reference angle is the size angle, [latex]t,[/latex] formed by the terminal side of the angle [latex]t[/latex] and the horizontal axis.
- Reference angles can be used to find the sine and cosine of the original angle.
- Reference angles can also be used to find the coordinates of a point on a unit circle.
- When the radius of a circle centered at the origin is not 1, we can find coordinates of a point on the circle by multiplying the sine and cosine of the angle by [latex]r.[/latex]
Glossary
- cosine function
- the x-value of the point on a unit circle corresponding to a given angle
- Pythagorean Identity
- a corollary of the Pythagorean Theorem stating that the square of the cosine of a given angle plus the square of the sine of that angle equals 1
- sine function
- the y-value of the point on a unit circle corresponding to a given angle
- unit circle
- a circle with a center at [latex]\left(0,0\right)[/latex] and radius 1.
Candela Citations
- Unit Circle: Sine and Cosine Functions. Authored by: Douglas Hoffman. Provided by: Openstax. Located at: https://cnx.org/contents/8si1Yf2B@2.21:F6bX9ckM@7/Unit-Circle-Sine-and-Cosine-Functions. Project: Essential Precalcus, Part 2. License: CC BY: Attribution