Periodic Functions

# 40 Graphs of the Other Trigonometric Functions

### Learning Objectives

In this section, you will:

- Analyze the graph of y=tan x.
- Graph variations of y=tan x.
- Analyze the graphs of y=sec x and y=csc x.
- Graph variations of y=sec x and y=csc x.
- Analyze the graph of y=cot x.
- Graph variations of y=cot x.

We know the tangent function can be used to find distances, such as the height of a building, mountain, or flagpole. But what if we want to measure repeated occurrences of distance? Imagine, for example, a police car parked next to a warehouse. The rotating light from the police car would travel across the wall of the warehouse in regular intervals. If the input is time, the output would be the distance the beam of light travels. The beam of light would repeat the distance at regular intervals. The tangent function can be used to approximate this distance. Asymptotes would be needed to illustrate the repeated cycles when the beam runs parallel to the wall because, seemingly, the beam of light could appear to extend forever. The graph of the tangent function would clearly illustrate the repeated intervals. In this section, we will explore the graphs of the tangent and other trigonometric functions.

### Analyzing the Graph of *y* = tan *x*

We will begin with the graph of the tangent function, plotting points as we did for the sine and cosine functions. Recall that

The period of the tangent function isbecause the graph repeats itself on intervals ofwhereis a constant. If we graph the tangent function ontowe can see the behavior of the graph on one complete cycle. If we look at any larger interval, we will see that the characteristics of the graph repeat.

We can determine whether tangent is an odd or even function by using the definition of tangent.

Therefore, tangent is an odd function. We can further analyze the graphical behavior of the tangent function by looking at values for some of the special angles, as listed in (Figure).

0 | |||||||||

undefined | –1 | 0 | 1 | undefined |

These points will help us draw our graph, but we need to determine how the graph behaves where it is undefined. If we look more closely at values whenwe can use a table to look for a trend. Becauseandwe will evaluateat radian measuresas shown in (Figure).

1.3 | 1.5 | 1.55 | 1.56 | |

3.6 | 14.1 | 48.1 | 92.6 |

Asapproachesthe outputs of the function get larger and larger. Becauseis an odd function, we see the corresponding table of negative values in (Figure).

−1.3 | −1.5 | −1.55 | −1.56 | |

−3.6 | −14.1 | −48.1 | −92.6 |

We can see that, asapproachesthe outputs get smaller and smaller. Remember that there are some values offor whichFor example,andAt these values, the tangent function is undefined, so the graph ofhas discontinuities atAt these values, the graph of the tangent has vertical asymptotes. (Figure) represents the graph ofThe tangent is positive from 0 toand fromtocorresponding to quadrants I and III of the unit circle.

### Graphing Variations of *y* = tan *x*

As with the sine and cosine functions, the tangent function can be described by a general equation.

We can identify horizontal and vertical stretches and compressions using values ofandThe horizontal stretch can typically be determined from the period of the graph. With tangent graphs, it is often necessary to determine a vertical stretch using a point on the graph.

Because there are no maximum or minimum values of a tangent function, the term *amplitude* cannot be interpreted as it is for the sine and cosine functions. Instead, we will use the phrase *stretching/compressing factor* when referring to the constant

*y*=

*A*tan(

*Bx*)

- The stretching factor is
- The period is
- The domain is all real numberswheresuch thatis an integer.
- The range is
- The asymptotes occur atwhereis an integer.
- is an odd function.

#### Graphing One Period of a Stretched or Compressed Tangent Function

We can use what we know about the properties of the tangent function to quickly sketch a graph of any stretched and/or compressed tangent function of the formWe focus on a single period of the function including the origin, because the periodic property enables us to extend the graph to the rest of the function’s domain if we wish. Our limited domain is then the intervaland the graph has vertical asymptotes atwhereOnthe graph will come up from the left asymptote atcross through the origin, and continue to increase as it approaches the right asymptote atTo make the function approach the asymptotes at the correct rate, we also need to set the vertical scale by actually evaluating the function for at least one point that the graph will pass through. For example, we can use

because

**Given the functiongraph one period.**

- Identify the stretching factor,
- Identifyand determine the period,
- Draw vertical asymptotes atand
- Forthe graph approaches the left asymptote at negative output values and the right asymptote at positive output values (reverse for).
- Plot reference points atandand draw the graph through these points.

Sketch a graph of one period of the function

First, we identifyand

Becauseandwe can find the stretching/compressing factor and period. The period isso the asymptotes are atAt a quarter period from the origin, we have

This means the curve must pass through the pointsandThe only inflection point is at the origin. (Figure) shows the graph of one period of the function.

Sketch a graph of

#### Graphing One Period of a Shifted Tangent Function

Now that we can graph a tangent function that is stretched or compressed, we will add a vertical and/or horizontal (or phase) shift. In this case, we addandto the general form of the tangent function.

The graph of a transformed tangent function is different from the basic tangent functionin several ways:

*y*=

*A*tan(

*Bx*−

*C*)+

*D*

- The stretching factor is
- The period is
- The domain iswhereis an integer.
- The range is
- The vertical asymptotes occur atwhereis an odd integer.
- There is no amplitude.
- is an odd function because it is the quotient of odd and even functions (sine and cosine respectively).

**Given the functionsketch the graph of one period.**

- Express the function given in the form
- Identify the stretching/compressing factor,
- Identifyand determine the period,
- Identifyand determine the phase shift,
- Draw the graph ofshifted to the right byand up by
- Sketch the vertical asymptotes, which occur atwhereis an odd integer.
- Plot any three reference points and draw the graph through these points.

Graph one period of the function

*Step 1.*The function is already written in the form*Step 2.*so the stretching factor is*Step 3.*so the period is*Step 4.*so the phase shift is*Step 5-7.*The asymptotes are atandand the three recommended reference points areandThe graph is shown in (Figure).

Note that this is a decreasing function because

How would the graph in (Figure) look different if we madeinstead of

It would be reflected across the linebecoming an increasing function.

**Given the graph of a tangent function, identify horizontal and vertical stretches.**

- Find the periodfrom the spacing between successive vertical asymptotes or
*x*-intercepts. - Write
- Determine a convenient pointon the given graph and use it to determine

The graph has the shape of a tangent function.

*Step 1.*One cycle extends from –4 to 4, so the period isSincewe have*Step 2.*The equation must have the form*Step 3.*To find the vertical stretchwe can use the point

Because

This function would have a formula

Find a formula for the function in (Figure).

### Analyzing the Graphs of *y* = sec *x* and *y* = csc*x*

The secant was defined by the reciprocal identityNotice that the function is undefined when the cosine is 0, leading to vertical asymptotes atetc. Because the cosine is never more than 1 in absolute value, the secant, being the reciprocal, will never be less than 1 in absolute value.

We can graphby observing the graph of the cosine function because these two functions are reciprocals of one another. See (Figure). The graph of the cosine is shown as a dashed orange wave so we can see the relationship. Where the graph of the cosine function decreases, the graph of the secant function increases. Where the graph of the cosine function increases, the graph of the secant function decreases. When the cosine function is zero, the secant is undefined.

The secant graph has vertical asymptotes at each value ofwhere the cosine graph crosses the *x*-axis; we show these in the graph below with dashed vertical lines, but will not show all the asymptotes explicitly on all later graphs involving the secant and cosecant.

Note that, because cosine is an even function, secant is also an even function. That is,

As we did for the tangent function, we will again refer to the constantas the stretching factor, not the amplitude.

*y*=

*A*sec(

*Bx*)

- The stretching factor is
- The period is
- The domain iswhereis an odd integer.
- The range is
- The vertical asymptotes occur atwhereis an odd integer.
- There is no amplitude.
- is an even function because cosine is an even function.

Similar to the secant, the cosecant is defined by the reciprocal identityNotice that the function is undefined when the sine is 0, leading to a vertical asymptote in the graph atetc. Since the sine is never more than 1 in absolute value, the cosecant, being the reciprocal, will never be less than 1 in absolute value.

We can graphby observing the graph of the sine function because these two functions are reciprocals of one another. See (Figure). The graph of sine is shown as a dashed orange wave so we can see the relationship. Where the graph of the sine function decreases, the graph of the cosecant function increases. Where the graph of the sine function increases, the graph of the cosecant function decreases.

The cosecant graph has vertical asymptotes at each value ofwhere the sine graph crosses the *x*-axis; we show these in the graph below with dashed vertical lines.

Note that, since sine is an odd function, the cosecant function is also an odd function. That is,

The graph of cosecant, which is shown in (Figure), is similar to the graph of secant.

*y*=

*A*csc(

*Bx*)

- The stretching factor is
- The period is
- The domain iswhereis an integer.
- The range is
- The asymptotes occur atwhereis an integer.
- is an odd function because sine is an odd function.

### Graphing Variations of *y* = sec *x* and *y*= csc *x*

For shifted, compressed, and/or stretched versions of the secant and cosecant functions, we can follow similar methods to those we used for tangent and cotangent. That is, we locate the vertical asymptotes and also evaluate the functions for a few points (specifically the local extrema). If we want to graph only a single period, we can choose the interval for the period in more than one way. The procedure for secant is very similar, because the cofunction identity means that the secant graph is the same as the cosecant graph shifted half a period to the left. Vertical and phase shifts may be applied to the cosecant function in the same way as for the secant and other functions.The equations become the following.

*y*=

*A*sec(

*Bx*−

*C*)+

*D*

- The stretching factor is
- The period is
- The domain iswhereis an odd integer.
- The range is
- The vertical asymptotes occur atwhereis an odd integer.
- There is no amplitude.
- is an even function because cosine is an even function.

*y*=

*A*csc(

*Bx*−

*C*)+

*D*

- The stretching factor is
- The period is
- The domain iswhereis an integer.
- The range is
- The vertical asymptotes occur atwhereis an integer.
- There is no amplitude.
- is an odd function because sine is an odd function.

**Given a function of the formgraph one period.**

- Express the function given in the form
- Identify the stretching/compressing factor,
- Identifyand determine the period,
- Sketch the graph of
- Use the reciprocal relationship betweenandto draw the graph of
- Sketch the asymptotes.
- Plot any two reference points and draw the graph through these points.

Graph one period of

*Step 1.*The given function is already written in the general form,*Step 2.*so the stretching factor is*Step 3.*soThe period isunits.*Step 4.*Sketch the graph of the function*Step 5.*Use the reciprocal relationship of the cosine and secant functions to draw the cosecant function.*Steps 6–7.*Sketch two asymptotes atandWe can use two reference points, the local minimum atand the local maximum at(Figure) shows the graph.

Graph one period of

This is a vertical reflection of the preceding graph becauseis negative.

**Do the vertical shift and stretch/compression affect the secant’s range?**

*Yes. The range of**is*

**Given a function of the formgraph one period.**

- Express the function given in the form
- Identify the stretching/compressing factor,
- Identifyand determine the period,
- Identifyand determine the phase shift,
- Draw the graph of, but shift it to the right byand up by
- Sketch the vertical asymptotes, which occur atwhereis an odd integer.

Graph one period of

*Step 1.*Express the function given in the form*Step 2.*The stretching/compressing factor is*Step 3.*The period is*Step 4.*The phase shift is*Step 5.*Draw the graph of but shift it to the right byand up by*Step 6.*Sketch the vertical asymptotes, which occur atandThere is a local minimum atand a local maximum at(Figure) shows the graph.

Graph one period of

**The domain ofwas given to be allsuch thatfor any integer Would the domain of**

*Yes. The excluded points of the domain follow the vertical asymptotes. Their locations show the horizontal shift and compression or expansion implied by the transformation to the original function’s input.*

**Given a function of the formgraph one period.**

- Express the function given in the form
- Identifyand determine the period,
- Draw the graph of
- Use the reciprocal relationship betweenandto draw the graph of
- Sketch the asymptotes.
- Plot any two reference points and draw the graph through these points.

Graph one period of

*Step 1.*The given function is already written in the general form,*Step 2.*so the stretching factor is 3.*Step 3.*soThe period isunits.*Step 4.*Sketch the graph of the function*Step 5.*Use the reciprocal relationship of the sine and cosecant functions to draw the cosecant function.*Steps 6–7.*Sketch three asymptotes atandWe can use two reference points, the local maximum atand the local minimum at(Figure) shows the graph.

Graph one period of

**Given a function of the formgraph one period.**

- Express the function given in the form
- Identify the stretching/compressing factor,
- Identifyand determine the period,
- Identifyand determine the phase shift,
- Draw the graph ofbut shift it to the right by and up by
- Sketch the vertical asymptotes, which occur atwhereis an integer.

Sketch a graph ofWhat are the domain and range of this function?

*Step 1.*Express the function given in the form*Step 2.*Identify the stretching/compressing factor,*Step 3.*The period is*Step 4.*The phase shift is*Step 5.*Draw the graph ofbut shift it up*Step 6.*Sketch the vertical asymptotes, which occur at

The graph for this function is shown in (Figure).

The vertical asymptotes shown on the graph mark off one period of the function, and the local extrema in this interval are shown by dots. Notice how the graph of the transformed cosecant relates to the graph ofshown as the orange dashed wave.

Given the graph ofshown in (Figure), sketch the graph ofon the same axes.

### Analyzing the Graph of *y* = cot *x*

The last trigonometric function we need to explore is cotangent. The cotangent is defined by the reciprocal identityNotice that the function is undefined when the tangent function is 0, leading to a vertical asymptote in the graph atetc. Since the output of the tangent function is all real numbers, the output of the cotangent function is also all real numbers.

We can graphby observing the graph of the tangent function because these two functions are reciprocals of one another. See (Figure). Where the graph of the tangent function decreases, the graph of the cotangent function increases. Where the graph of the tangent function increases, the graph of the cotangent function decreases.

The cotangent graph has vertical asymptotes at each value ofwherewe show these in the graph below with dashed lines. Since the cotangent is the reciprocal of the tangent,has vertical asymptotes at all values ofwhereandat all values ofwherehas its vertical asymptotes.

*y*=

*A*cot(

*Bx*)

- The stretching factor is
- The period is
- The domain iswhereis an integer.
- The range is
- The asymptotes occur atwhereis an integer.
- is an odd function.

### Graphing Variations of *y* = cot *x*

We can transform the graph of the cotangent in much the same way as we did for the tangent. The equation becomes the following.

*y*=

*A*cot(

*Bx*−C)+

*D*

- The stretching factor is
- The period is
- The domain iswhereis an integer.
- The range is
- The vertical asymptotes occur atwhereis an integer.
- There is no amplitude.
- is an odd function because it is the quotient of even and odd functions (cosine and sine, respectively)

**Given a modified cotangent function of the formgraph one period.**

- Express the function in the form
- Identify the stretching factor,
- Identify the period,
- Draw the graph of
- Plot any two reference points.
- Use the reciprocal relationship between tangent and cotangent to draw the graph of
- Sketch the asymptotes.

Determine the stretching factor, period, and phase shift ofand then sketch a graph.

*Step 1.*Expressing the function in the formgives*Step 2.*The stretching factor is*Step 3.*The period is*Step 4.*Sketch the graph of*Step 5.*Plot two reference points. Two such points areand*Step 6.*Use the reciprocal relationship to draw*Step 7.*Sketch the asymptotes,

The orange graph in (Figure) showsand the blue graph shows

**Given a modified cotangent function of the formgraph one period.**

- Express the function in the form
- Identify the stretching factor,
- Identify the period,
- Identify the phase shift,
- Draw the graph of shifted to the right byand up by
- Sketch the asymptoteswhereis an integer.
- Plot any three reference points and draw the graph through these points.

Sketch a graph of one period of the function

*Step 1.*The function is already written in the general form*Step 2.*so the stretching factor is 4.*Step 3.*so the period is*Step 4.*so the phase shift is*Step 5.*We draw*Step 6-7.*Three points we can use to guide the graph areandWe use the reciprocal relationship of tangent and cotangent to draw*Step 8.*The vertical asymptotes areand

The graph is shown in (Figure).

### Using the Graphs of Trigonometric Functions to Solve Real-World Problems

Many real-world scenarios represent periodic functions and may be modeled by trigonometric functions. As an example, let’s return to the scenario from the section opener. Have you ever observed the beam formed by the rotating light on a police car and wondered about the movement of the light beam itself across the wall? The periodic behavior of the distance the light shines as a function of time is obvious, but how do we determine the distance? We can use the tangent function.

Suppose the functionmarks the distance in the movement of a light beam from the top of a police car across a wall whereis the time in seconds andis the distance in feet from a point on the wall directly across from the police car.

- Find and interpret the stretching factor and period.
- Graph on the interval
- Evaluateand discuss the function’s value at that input.

- We know from the general form ofthatis the stretching factor andis the period.
We see that the stretching factor is 5. This means that the beam of light will have moved 5 ft after half the period.

The period isThis means that every 4 seconds, the beam of light sweeps the wall. The distance from the spot across from the police car grows larger as the police car approaches.

- To graph the function, we draw an asymptote atand use the stretching factor and period. See (Figure)
- period:after 1 second, the beam of has moved 5 ft from the spot across from the police car.

Access these online resources for additional instruction and practice with graphs of other trigonometric functions.

### Key Equations

Shifted, compressed, and/or stretched tangent function | |

Shifted, compressed, and/or stretched secant function | |

Shifted, compressed, and/or stretched cosecant function | |

Shifted, compressed, and/or stretched cotangent function |

### Key Concepts

- The tangent function has period
- is a tangent with vertical and/or horizontal stretch/compression and shift. See (Figure), (Figure), and (Figure).
- The secant and cosecant are both periodic functions with a period ofgives a shifted, compressed, and/or stretched secant function graph. See (Figure) and (Figure).
- gives a shifted, compressed, and/or stretched cosecant function graph. See (Figure) and (Figure).
- The cotangent function has periodand vertical asymptotes at
- The range of cotangent isand the function is decreasing at each point in its range.
- The cotangent is zero at
- is a cotangent with vertical and/or horizontal stretch/compression and shift. See (Figure) and (Figure).
- Real-world scenarios can be solved using graphs of trigonometric functions. See (Figure).

### Section Exercises

#### Verbal

Explain how the graph of the sine function can be used to graph

Sinceis the reciprocal function ofyou can plot the reciprocal of the coordinates on the graph ofto obtain the *y*-coordinates ofThe *x*-intercepts of the graphare the vertical asymptotes for the graph of

How can the graph ofbe used to construct the graph of

Explain why the period ofis equal to

Answers will vary. Using the unit circle, one can show that

Why are there no intercepts on the graph of

How does the period ofcompare with the period of

The period is the same:

#### Algebraic

For the following exercises, match each trigonometric function with one of the following graphs.

IV

III

For the following exercises, find the period and horizontal shift of each of the functions.

period: 8; horizontal shift: 1 unit to left

Iffind

1.5

Iffind

Iffind

5

Iffind

For the following exercises, rewrite each expression such that the argumentis positive.

#### Graphical

For the following exercises, sketch two periods of the graph for each of the following functions. Identify the stretching factor, period, and asymptotes.

stretching factor: 2; period:asymptotes:

stretching factor: 6; period: 6; asymptotes:

stretching factor: 1; period:asymptotes:

Stretching factor: 1; period:asymptotes:

stretching factor: 2; period:asymptotes:

stretching factor: 4; period:asymptotes:

stretching factor: 7; period:asymptotes:

stretching factor: 2; period:asymptotes:

stretching factor:period:asymptotes:

For the following exercises, find and graph two periods of the periodic function with the given stretching factor,period, and phase shift.

A tangent curve,period ofand phase shift

A tangent curve,period ofand phase shift

For the following exercises, find an equation for the graph of each function.

#### Technology

For the following exercises, use a graphing calculator to graph two periods of the given function. Note: most graphing calculators do not have a cosecant button; therefore, you will need to inputas

GraphWhat is the function shown in the graph?

#### Real-World Applications

The functionmarks the distance in the movement of a light beam from a police car across a wall for timein seconds, and distance in feet.

- Graph on the interval
- Find and interpret the stretching factor, period, and asymptote.
- Evaluateandand discuss the function’s values at those inputs.

Standing on the shore of a lake, a fisherman sights a boat far in the distance to his left. Letmeasured in radians, be the angle formed by the line of sight to the ship and a line due north from his position. Assume due north is 0 andis measured negative to the left and positive to the right. (See (Figure).) The boat travels from due west to due east and, ignoring the curvature of the Earth, the distancein kilometers, from the fisherman to the boat is given by the function

- What is a reasonable domain for
- Graphon this domain.
- Find and discuss the meaning of any vertical asymptotes on the graph of
- Calculate and interpretRound to the second decimal place.
- Calculate and interpretRound to the second decimal place.
- What is the minimum distance between the fisherman and the boat? When does this occur?

- andthe distance grows without bound asapproaches—i.e., at right angles to the line representing due north, the boat would be so far away, the fisherman could not see it;
- 3; whenthe boat is 3 km away;
- 1.73; whenthe boat is about 1.73 km away;
- 1.5 km; when

A laser rangefinder is locked on a comet approaching Earth. The distancein kilometers, of the comet afterdays, forin the interval 0 to 30 days, is given by

- Graphon the interval
- Evaluate and interpret the information.
- What is the minimum distance between the comet and Earth? When does this occur? To which constant in the equation does this correspond?
- Find and discuss the meaning of any vertical asymptotes.

A video camera is focused on a rocket on a launching pad 2 miles from the camera. The angle of elevation from the ground to the rocket afterseconds is

- Write a function expressing the altitudein miles, of the rocket above the ground afterseconds. Ignore the curvature of the Earth.
- Graphon the interval
- Evaluate and interpret the valuesand
- What happens to the values ofas approaches 60 seconds? Interpret the meaning of this in terms of the problem.

- after 0 seconds, the rocket is 0 mi above the ground;after 30 seconds, the rockets is 2 mi high;
- Asapproaches 60 seconds, the values ofgrow increasingly large. The distance to the rocket is growing so large that the camera can no longer track it.