6.2: Graphs of the Sine and Cosine Functions
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- Graph variations of and .
- Use phase shifts of sine and cosine curves.
White light, such as the light from the sun, is not actually white at all. Instead, it is a composition of all the colors of the rainbow in the form of waves. The individual colors can be seen only when white light passes through an optical prism that separates the waves according to their wavelengths to form a rainbow.
Light waves can be represented graphically by the sine function. In the chapter on Trigonometric Functions, we examined trigonometric functions such as the sine function. In this section, we will interpret and create graphs of sine and cosine functions.
Graphing Sine and Cosine Functions
Recall that the sine and cosine functions relate real number values to the x- and y-coordinates of a point on the unit circle. So what do they look like on a graph on a coordinate plane? Let’s start with the sine function. We can create a table of values and use them to sketch a graph. Table 1 lists some of the values for the sine function on a unit circle.
Plotting the points from the table and continuing along the x-axis gives the shape of the sine function. See Figure 2.
Notice how the sine values are positive between 0 and which correspond to the values of the sine function in quadrants I and II on the unit circle, and the sine values are negative between and which correspond to the values of the sine function in quadrants III and IV on the unit circle. See Figure 3.
Now let’s take a similar look at the cosine function. Again, we can create a table of values and use them to sketch a graph. Table 2 lists some of the values for the cosine function on a unit circle.
As with the sine function, we can plots points to create a graph of the cosine function as in Figure 4.
Because we can evaluate the sine and cosine of any real number, both of these functions are defined for all real numbers. By thinking of the sine and cosine values as coordinates of points on a unit circle, it becomes clear that the range of both functions must be the interval
In both graphs, the shape of the graph repeats after which means the functions are periodic with a period of A periodic function is a function for which a specific horizontal shift, P, results in a function equal to the original function: for all values of in the domain of When this occurs, we call the smallest such horizontal shift with the period of the function. Figure 5 shows several periods of the sine and cosine functions.
Looking again at the sine and cosine functions on a domain centered at the y-axis helps reveal symmetries. As we can see in Figure 6, the sine function is symmetric about the origin. Recall from The Other Trigonometric Functions that we determined from the unit circle that the sine function is an odd function because Now we can clearly see this property from the graph.
Figure 7 shows that the cosine function is symmetric about the y-axis. Again, we determined that the cosine function is an even function. Now we can see from the graph that
Characteristics of Sine and Cosine Functions
The sine and cosine functions have several distinct characteristics:
- They are periodic functions with a period of
- The domain of each function is and the range is
- The graph of is symmetric about the origin, because it is an odd function.
- The graph of is symmetric about the -axis, because it is an even function.
Investigating Sinusoidal Functions
As we can see, sine and cosine functions have a regular period and range. If we watch ocean waves or ripples on a pond, we will see that they resemble the sine or cosine functions. However, they are not necessarily identical. Some are taller or longer than others. A function that has the same general shape as a sine or cosine function is known as a sinusoidal function. The general forms of sinusoidal functions are
Determining the Period of Sinusoidal Functions
Looking at the forms of sinusoidal functions, we can see that they are transformations of the sine and cosine functions. We can use what we know about transformations to determine the period.
In the general formula, is related to the period by If then the period is less than and the function undergoes a horizontal compression, whereas if then the period is greater than and the function undergoes a horizontal stretch. For example, so the period is which we knew. If then so the period is and the graph is compressed. If then so the period is and the graph is stretched. Notice in Figure 8 how the period is indirectly related to
Period of Sinusoidal Functions
If we let and in the general form equations of the sine and cosine functions, we obtain the forms
The period is
Example 1
Identifying the Period of a Sine or Cosine Function
Determine the period of the function
- Answer
Let’s begin by comparing the equation to the general form
In the given equation, so the period will be
Try It #1
Determine the period of the function
Determining Amplitude
Returning to the general formula for a sinusoidal function, we have analyzed how the variable relates to the period. Now let’s turn to the variable so we can analyze how it is related to the amplitude, or greatest distance from rest. represents the vertical stretch factor, and its absolute value is the amplitude. The local maxima will be a distance above the horizontal midline of the graph, which is the line because in this case, the midline is the x-axis. The local minima will be the same distance below the midline. If the function is stretched. For example, the amplitude of is twice the amplitude of If the function is compressed. Figure 9 compares several sine functions with different amplitudes.
Amplitude of Sinusoidal Functions
If we let and in the general form equations of the sine and cosine functions, we obtain the forms
The amplitude is which is the vertical height from the midline In addition, notice in the example that
Example 2
Identifying the Amplitude of a Sine or Cosine Function
What is the amplitude of the sinusoidal function Is the function stretched or compressed vertically?
- Answer
Let’s begin by comparing the function to the simplified form
In the given function, so the amplitude is The function is stretched.
Analysis
The negative value of results in a reflection across the x-axis of the sine function, as shown in Figure 10.
Try It #2
What is the amplitude of the sinusoidal function Is the function stretched or compressed vertically?
Analyzing Graphs of Variations of y = sin x and y = cos x
Now that we understand how and relate to the general form equation for the sine and cosine functions, we will explore the variables and Recall the general form:
The value for a sinusoidal function is called the phase shift, or the horizontal displacement of the basic sine or cosine function. If the graph shifts to the right. If the graph shifts to the left. The greater the value of the more the graph is shifted. Figure 11 shows that the graph of shifts to the right by units, which is more than we see in the graph of which shifts to the right by units.
While relates to the horizontal shift, indicates the vertical shift from the midline in the general formula for a sinusoidal function. See Figure 12. The function has its midline at
Any value of other than zero shifts the graph up or down. Figure 13 compares with which is shifted 2 units up on a graph.
Variations of Sine and Cosine Functions
Given an equation in the form or is the phase shift and is the vertical shift.
Example 3
Identifying the Phase Shift of a Function
Determine the direction and magnitude of the phase shift for
- Answer
Let’s begin by comparing the equation to the general form
In the given equation, notice that and So the phase shift is
or units to the left.
Analysis
We must pay attention to the sign in the equation for the general form of a sinusoidal function. The equation shows a minus sign before Therefore can be rewritten as If the value of is negative, the shift is to the left.
Try It #3
Determine the direction and magnitude of the phase shift for
Example 4
Identifying the Vertical Shift of a Function
Determine the direction and magnitude of the vertical shift for
- Answer
Let’s begin by comparing the equation to the general form
In the given equation, so the shift is 3 units downward.
Try It #4
Determine the direction and magnitude of the vertical shift for
How To
Given a sinusoidal function in the form identify the midline, amplitude, period, and phase shift.
- Determine the amplitude as
- Determine the period as
- Determine the phase shift as
- Determine the midline as
Example 5
Identifying the Variations of a Sinusoidal Function from an Equation
Determine the midline, amplitude, period, and phase shift of the function
- Answer
Let’s begin by comparing the equation to the general form
so the amplitude is
Next, so the period is
There is no added constant inside the parentheses, so and the phase shift is
Finally, so the midline is
Analysis
Inspecting the graph, we can determine that the period is the midline is and the amplitude is 3. See Figure 14.
Try It #5
Determine the midline, amplitude, period, and phase shift of the function
Example 6
Identifying the Equation for a Sinusoidal Function from a Graph
Determine the formula for the cosine function in Figure 15.
- Answer
To determine the equation, we need to identify each value in the general form of a sinusoidal function.
The graph could represent either a sine or a cosine function that is shifted and/or reflected. When the graph has an extreme point, Since the cosine function has an extreme point for let us write our equation in terms of a cosine function.
Let’s start with the midline. We can see that the graph rises and falls an equal distance above and below This value, which is the midline, is in the equation, so
The greatest distance above and below the midline is the amplitude. The maxima are 0.5 units above the midline and the minima are 0.5 units below the midline. So Another way we could have determined the amplitude is by recognizing that the difference between the height of local maxima and minima is 1, so Also, the graph is reflected about the x-axis so that
The graph is not horizontally stretched or compressed, so and the graph is not shifted horizontally, so
Putting this all together,
Try It #6
Determine the formula for the sine function in Figure 16.
Example 7
Identifying the Equation for a Sinusoidal Function from a Graph
Determine the equation for the sinusoidal function in Figure 17.
- Answer
With the highest value at 1 and the lowest value at the midline will be halfway between at So
The distance from the midline to the highest or lowest value gives an amplitude of
The period of the graph is 6, which can be measured from the peak at to the next peak at or from the distance between the lowest points. Therefore, Using the positive value for we find that
So far, our equation is either or For the shape and shift, we have more than one option. We could write this as any one of the following:
- a cosine shifted to the right
- a negative cosine shifted to the left
- a sine shifted to the left
- a negative sine shifted to the right
While any of these would be correct, the cosine shifts are easier to work with than the sine shifts in this case because they involve integer values. So our function becomes
Again, these functions are equivalent, so both yield the same graph.
Try It #7
Write a formula for the function graphed in Figure 18.
Graphing Variations of y = sin x and y = cos x
Throughout this section, we have learned about types of variations of sine and cosine functions and used that information to write equations from graphs. Now we can use the same information to create graphs from equations.
Instead of focusing on the general form equations
we will let and and work with a simplified form of the equations in the following examples.
How To
Given the function sketch its graph.
- Identify the amplitude,
- Identify the period,
- Start at the origin, with the function increasing to the right if is positive or decreasing if is negative.
- At there is a local maximum for or a minimum for with
- The curve returns to the x-axis at
- There is a local minimum for (maximum for ) at with
- The curve returns again to the x-axis at
Example 8
Graphing a Function and Identifying the Amplitude and Period
Sketch a graph of
- Answer
Let’s begin by comparing the equation to the form
- Step 1. We can see from the equation that so the amplitude is 2.
- Step 2. The equation shows that so the period is
- Step 3. Because is negative, the graph descends as we move to the right of the origin.
- Step 4–7. The x-intercepts are at the beginning of one period, the horizontal midpoints are at and at the end of one period at
The quarter points include the minimum at and the maximum at A local minimum will occur 2 units below the midline, at and a local maximum will occur at 2 units above the midline, at Figure 19 shows the graph of the function.
Try It #8
Sketch a graph of Determine the midline, amplitude, period, and phase shift.
How To
Given a sinusoidal function with a phase shift and a vertical shift, sketch its graph.
- Express the function in the general form
- Identify the amplitude,
- Identify the period,
- Identify the phase shift,
- Draw the graph of shifted to the right or left by and up or down by
Example 9
Graphing a Transformed Sinusoid
Sketch a graph of
- Answer
- Step 1. The function is already written in general form: This graph will have the shape of a sine function, starting at the midline and increasing to the right.
- Step 2. The amplitude is 3.
- Step 3. Since we determine the period as follows.
The period is 8.
- Step 4. Since the phase shift is
The phase shift is 1 unit.
- Step 5. Figure 20 shows the graph of the function.
Try It #9
Draw a graph of Determine the midline, amplitude, period, and phase shift.
Example 10
Identifying the Properties of a Sinusoidal Function
Given determine the amplitude, period, phase shift, and vertical shift. Then graph the function.
- Answer
Begin by comparing the equation to the general form and use the steps outlined in Example 9.
- Step 1. The function is already written in general form.
- Step 2. Since the amplitude is
- Step 3. so the period is The period is 4.
- Step 4. so we calculate the phase shift as The phase shift is
- Step 5. so the midline is and the vertical shift is up 3.
Since is negative, the graph of the cosine function has been reflected about the x-axis.
Figure 21 shows one cycle of the graph of the function.
Using Transformations of Sine and Cosine Functions
We can use the transformations of sine and cosine functions in numerous applications. As mentioned at the beginning of the chapter, circular motion can be modeled using either the sine or cosine function.
Example 11
Finding the Vertical Component of Circular Motion
A point rotates around a circle of radius 3 centered at the origin. Sketch a graph of the y-coordinate of the point as a function of the angle of rotation.
- Answer
Recall that, for a point on a circle of radius r, the y-coordinate of the point is so in this case, we get the equation The constant 3 causes a vertical stretch of the y-values of the function by a factor of 3, which we can see in the graph in Figure 22.
Analysis
Notice that the period of the function is still as we travel around the circle, we return to the point for Because the outputs of the graph will now oscillate between and the amplitude of the sine wave is
Try It #10
What is the amplitude of the function Sketch a graph of this function.
Example 12
Finding the Vertical Component of Circular Motion
A circle with radius 3 ft is mounted with its center 4 ft off the ground. The point closest to the ground is labeled P, as shown in Figure 23. Sketch a graph of the height above the ground of the point as the circle is rotated; then find a function that gives the height in terms of the angle of rotation.
- Answer
Sketching the height, we note that it will start 1 ft above the ground, then increase up to 7 ft above the ground, and continue to oscillate 3 ft above and below the center value of 4 ft, as shown in Figure 24.
Although we could use a transformation of either the sine or cosine function, we start by looking for characteristics that would make one function easier to use than the other. Let’s use a cosine function because it starts at the highest or lowest value, while a sine function starts at the middle value. A standard cosine starts at the highest value, and this graph starts at the lowest value, so we need to incorporate a vertical reflection.
Second, we see that the graph oscillates 3 above and below the center, while a basic cosine has an amplitude of 1, so this graph has been vertically stretched by 3, as in the last example.
Finally, to move the center of the circle up to a height of 4, the graph has been vertically shifted up by 4. Putting these transformations together, we find that
Try It #11
A weight is attached to a spring that is then hung from a board, as shown in Figure 25. As the spring oscillates up and down, the position of the weight relative to the board ranges from in. (at time to in. (at time below the board. Assume the position of is given as a sinusoidal function of Sketch a graph of the function, and then find a cosine function that gives the position in terms of
Example 13
Determining a Rider’s Height on a Ferris Wheel
The London Eye is a huge Ferris wheel with a diameter of 135 meters (443 feet). It completes one rotation every 30 minutes. Riders board from a platform 2 meters above the ground. Express a rider’s height above ground as a function of time in minutes.
- Answer
With a diameter of 135 m, the wheel has a radius of 67.5 m. The height will oscillate with amplitude 67.5 m above and below the center.
Passengers board 2 m above ground level, so the center of the wheel must be located m above ground level. The midline of the oscillation will be at 69.5 m.
The wheel takes 30 minutes to complete 1 revolution, so the height will oscillate with a period of 30 minutes.
Lastly, because the rider boards at the lowest point, the height will start at the smallest value and increase, following the shape of a vertically reflected cosine curve.
- Amplitude: so
- Midline: so
- Period: so
- Shape:
An equation for the rider’s height would be
where is in minutes and is measured in meters.
Media
Access these online resources for additional instruction and practice with graphs of sine and cosine functions.
6.1 Section Exercises
Verbal
Why are the sine and cosine functions called periodic functions?
How does the graph of compare with the graph of Explain how you could horizontally translate the graph of to obtain
For the equation what constants affect the range of the function and how do they affect the range?
How does the range of a translated sine function relate to the equation
How can the unit circle be used to construct the graph of
Graphical
For the following exercises, graph two full periods of each function and state the amplitude, period, and midline. State the maximum and minimum y-values and their corresponding x-values on one period for Round answers to two decimal places if necessary.
For the following exercises, graph one full period of each function, starting at For each function, state the amplitude, period, and midline. State the maximum and minimum y-values and their corresponding x-values on one period for State the phase shift and vertical translation, if applicable. Round answers to two decimal places if necessary.
Determine the amplitude, midline, period, and an equation involving the sine function for the graph shown in Figure 26.
Determine the amplitude, period, midline, and an equation involving cosine for the graph shown in Figure 27.
Determine the amplitude, period, midline, and an equation involving cosine for the graph shown in Figure 28.
Determine the amplitude, period, midline, and an equation involving sine for the graph shown in Figure 29.
Determine the amplitude, period, midline, and an equation involving cosine for the graph shown in Figure 30.
Determine the amplitude, period, midline, and an equation involving sine for the graph shown in Figure 31.
Determine the amplitude, period, midline, and an equation involving cosine for the graph shown in Figure 32.
Determine the amplitude, period, midline, and an equation involving sine for the graph shown in Figure 33.
Algebraic
For the following exercises, let
On solve
On solve
Evaluate
On Find all values of
On the maximum value(s) of the function occur(s) at what x-value(s)?
On the minimum value(s) of the function occur(s) at what x-value(s)?
Show that This means that is an odd function and possesses symmetry with respect to ________________.
For the following exercises, let
On solve the equation
On solve
On find the x-intercepts of
On find the x-values at which the function has a maximum or minimum value.
On solve the equation
Technology
Graph on Explain why the graph appears as it does.
Graph on Did the graph appear as predicted in the previous exercise?
Graph on and verbalize how the graph varies from the graph of
Graph on the window and explain what the graph shows.
Graph on the window and explain what the graph shows.
Real-World Applications
A Ferris wheel is 25 meters in diameter and boarded from a platform that is 1 meter above the ground. The six o’clock position on the Ferris wheel is level with the loading platform. The wheel completes 1 full revolution in 10 minutes. The function gives a person’s height in meters above the ground t minutes after the wheel begins to turn.
- ⓐ Find the amplitude, midline, and period of
- ⓑ Find a formula for the height function
- ⓒ How high off the ground is a person after 5 minutes?