copyright © 2009 pearson addison-wesley 4.1-1 4 graphs of the circular functions

Copyright © 2009 Pearson Addison-Wesley 4.1-1

4Graphs of the Circular Functions


4.1 Graphs of the Sine and Cosine Functions

4.2 Translations of the Graphs of the Sine and Cosine Functions

4.3 Graphs of the Tangent and Cotangent Functions

4.4 Graphs of the Secant and Cosecant Functions

4.5Harmonic Motion

4Graphs of the Circular Functions

Copyright © 2009 Pearson Addison-Wesley

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Graphs of the Sine and Cosine Functions

4.1

Periodic Functions ▪ Graph of the Sine Function ▪ Graph of the Cosine Function ▪ Graphing Techniques, Amplitude, and Period ▪ Using a Trigonometric Model


Periodic Functions

Many things in daily life repeat with a predictable pattern, such as weather, tides, and hours of daylight.

This periodic graph represents a normal heartbeat.


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Periodic Function

A periodic function is a function f such that

f(x) = f(x + np),

for every real number x in the domain of f, every integer n, and some positive real number p. The least possible positive value of p is the period of the function.


Periodic Functions

The circumference of the unit circle is 2π, so the least possible value of p for which the sine and cosine functions repeat is 2π.

Therefore, the sine and cosine functions are periodic functions with period 2π.


Values of the Sine and Cosine Functions


Sine Function f(x) = sin x


Sine Function f(x) = sin x

The graph is continuous over its entire domain,(–, ).

Its x-intercepts are of the form nπ, where n is an integer.

Its period is 2π.

The graph is symmetric with respect to the origin, so the function is an odd function. For all x in the domain, sin(–x) = –sin(x).


Cosine Function f(x) = cos x


Cosine Function f(x) = cos x

The graph is continuous over its entire domain,(–, ).

Its x-intercepts are of the form , where n is an integer.

Its period is 2π.

The graph is symmetric with respect to the y-axis, so the function is an even function. For all x in the domain, cos(–x) = cos(x).


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Example 1 GRAPHING y = a sin x

and compare to the graph of y = sin x.

The shape of the graph is the same as the shape of y = sin x.

The range of y = 2 sin x is [–2, 2].


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Example 1 GRAPHING y = a sin x (continued)


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Amplitude

The graph of y = a sin x or y = a cos x, with a ≠ 0, will have the same shape as the graph of y = sin x or y = cos x, respectively, except with range [–|a|, |a|].

The amplitude is |a|.



No matter what the value of the ampitude, the periods of y = a sin x and y = a cos x are still 2π.

Now consider y = sin 2x.

One complete cycle occurs in π units.



Now consider y = sin 4x.

One complete cycle occurs in units.

In general, the graph of a function of the form y = sin bx or y = cos bx, for b > 0, will have a period different from 2π when b ≠ 1.


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Period

For b > 0, the graph of y = sin bx will have the resemble that of of y = sin x, but with

period

For b > 0, the graph of y = cos bx will have the resemble that of of y = cos x, but with

period



Divide the interval into four equal parts to

obtain the values for which sin bx or cos bx equal –1, 0, or 1.

These values give the minimum points, x-intercepts, and maximum points on the graph.

Find the midpoint of the interval by adding the x-values of the endpoints and dividing by 2. Then find the midpoints of the two intervals using the same procedure.


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Example 2 GRAPHING y = sin bx

Graph y = sin 2x and compare to the graph of y = sin x.

The coefficient of x is 2, so b = 2, and the period is

The endpoints are 0 and and the three points between the endpoints are


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Example 2 GRAPHING y = sin bx (continued)

The x-values are


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Example 2 GRAPHING y = sin bx (continued)


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Example 3 GRAPHING y = cos bx

Graph over one period.

The period is

The endpoints are 0 and and the three points

between the endpoints are


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Example 3 GRAPHING y = cos bx (continued)

The amplitude is 1.


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Guidelines for Sketching Graphs of Sine and Cosine Functions

Step 1 Find the period, Start with 0 on the x-axis, and lay off a distance of

Step 2 Divide the interval into four equal parts.

Step 3 Evaluate the function for each of the five x-values resulting from Step 2. The points will be maximum points, minimum points, and x-intercepts.


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Step 4 Plot the points found in Step 3, and join them with a sinusoidal curve having amplitude |a|.

Step 5 Draw the graph over additional periods as needed.


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Example 4 GRAPHING y = a sin bx

Graph y = –2 sin 3x over one period.

The coefficient of x is 3, so b = 3, and the period is

The function will be graphed over the interval

Step 1

Step 2

Divide the interval into four equal parts to get

the x-values


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Example 4 GRAPHING y = a sin bx (continued)

Make a table of values determined by the x-values from Step 2.

Step 3


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Example 4 GRAPHING y = a sin bx (continued)

Steps 4, 5

Plot the points

Join the points with a sinusoidal curve with amplitude 2. The graph can be extended by repeating the cycle.


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Note

When a is negative, the graph of y = a sin bx is the reflection across the x-axis of the graph of y = |a| sin bx.


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Example 5 GRAPHING y = a cos bx FOR b EQUAL TO A MULTIPLE OF π

Graph y = –3 cos πx over one period.

The function will be graphed over the interval [0, 2].

Step 1

Step 2

Since b = π, the period is

Divide the interval [0, 2] into four equal parts to get the

x-values


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Example 5 GRAPHING y = a cos bx FOR b EQUAL TO A MULTIPLE OF π (continued)

Step 3

Make a table of values determined by the x-values from Step 2.


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Example 5 GRAPHING y = a cos bx FOR b EQUAL TO A MULTIPLE OF π (continued)

Steps 4, 5

Join the points with a sinusoidal curve with amplitude |–3| = 3. The graph can be extended by repeating the cycle.

Plot the points


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Note

When b is an integer multiple of π, the x-intercepts of the graph are rational numbers.


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Example 6 INTERPRETING A SINE FUNCTION MODEL

The average temperature (in °F) at Mould Bay, Canada, can be approximated by the function

where x is the month and x = 1 corresponds to January, x = 2 corresponds to February, and so on.

(a) To observe the graph over a two-year interval and to see the maximum and minimum points, graph f in the window [0, 25] by [–45, 45].


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Example 6 INTERPRETING A SINE FUNCTION MODEL (continued)

The amplitude of the graph is 34 and the period is


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(b) What is the average temperature during the month of May?

May is month 5. Graph the function using a calculator, then find the value at x = 5.

Alternatively, use a calculator to compute f(5).


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(c) What would be an approximation for the average yearly temperature at Mould Bay?

From the graph, it appear that the average yearly temperature is about 0°F since the graph is centerd vertically about the line y = 0.


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Translations of the Graphs of the Sine and Cosine Functions

4.2

Horizontal Translations ▪ Vertical Translations ▪ Combinations of Translations ▪ Determining a Trigonometric Model Using Curve Fitting


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Horizontal Translations

The graph of the function defined by y = f(x – d) is translated horizontally compared to the graph of y = f(x).

The translation is d units to the right if d > 0 and |d| units to the left if d < 0.


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Horizontal Translations

In the function y = f(x – d), the expression x – d is called the argument.

A horizontal translation is called a phase shift.


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Example 1 GRAPHING y = sin (x – d)

Graph over one period.

Divide this interval into four equal parts:

To find an interval of one period, solve the three-part

inequality

Method 1


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Example 1 GRAPHING y = sin (x – d) (continued)

Make a table of values determined by the x-values.


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Join the corresponding points with a smooth curve.

The period is 2π, and the amplitude is 1.


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The argument indicates that the graph of

y = sin x will be translated units to the right.

Method 2


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Example 2 GRAPHING y = a cos (x – d)


Method 1


inequality


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Example 2 GRAPHING y = a cos (x – d) (continued)



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Join the corresponding points with a smooth curve.

The period is 2π, and the amplitude is 3.


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Method 2

so the phase

shift is units to the left.


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Example 3 GRAPHING y = a cos b(x – d)


Method 1


inequality


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Example 3 GRAPHING y = a cos b(x – d) (continued)



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Join the corresponding points with a smooth curve. Then graph an additional half-period to the left and to the right.


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Method 2

Write the expression in the form a cos b(x – d).

Then a = –2, b = 3, and

The amplitude is |–2| = 2, the period is and the phase shift is units to the left as compared to the graph of y = –2 cos 3x.


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Vertical Translations

The graph of the function defined by y = c + f(x) is translated vertically compared to the graph of y = f(x).

The translation is c units up if c > 0 and |c| units down if c < 0.


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Example 4 GRAPHING y = c + a cos bx

The graph of y = 3 – 2 cos 3x is the same as the graph of y = –2 cos 3x, vertically translated 3 units up.

The period of –2 cos 3x is so the key points have x-values


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Example 4 GRAPHING y = c + a cos bx (continued)

Make a table of points.


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Example 4 GRAPHING y = c + a cos bx (continued)

Join the corresponding points with a smooth curve. Then graph an additional period to the left.


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Method 1

Step 1 Find an interval whose length is one period by solving the three-part inequality

Step 3 Evaluate the function for each of the five x-values resulting from Step 2. The points will be maximum points, minimum points, and pointsthat intersect the line y = c.


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Step 4 Plot the points found in Step 3, and join them with a sinusoidal curve having amplitude |a|.

Step 5 Draw the graph over additional periods as needed.


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Method 2

First graph y = a sin bx or y = a cos bx. The amplitude of the function is |a|, and the period is

Use translations to graph the desired function. The vertical translation is c units up if c > 0 and |c| units down if c < 0.

The horizontal translation (phase shift) is d units to the right if d > 0 and |d| units to the left if d < 0.


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Example 5 GRAPHING y = c + a sin b(x – d)

Use Method 1:

Step 1: Find an interval whose length is one period.

Divide each part by 4.

Subtract from each part.


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Example 5 GRAPHING y = c + a sin b(x – d) (cont.)

Step 3: Make a table of values.

Step 2: Divide the interval into four equal parts to get the x-values.


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Steps 4 and 5: Plot the points found in the table and join them with a sinusoidal curve. Extend the graph an additional half-period to the left and to the right to include two full periods.


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Example 6 MODELING TEMPERATURE WITH A SINE FUNCTION

The maximum average monthly temperature in New Orleans is 82°F and the minimum is 54°F. The table shows the average monthly temperatures.


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Example 6 MODELING TEMPERATURE WITH A SINE FUNCTION (continued)

The scatter diagram for a two-year interval suggests that the temperatures can be modeled with a sine curve.

(a) Using only the maximum and minimum temperatures, determine a function of the form

where a, b, c, and d are constants, that models the average monthly temperature in New Orleans. Let x represent the month, with January corresponding to x = 1.


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Use the maximum and minimum average monthly temperatures to determine the amplitude a:

The average of the maximum and minimum temperatures gives c:

Since temperatures repeat every 12 months,


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To determine the phase shift, observe that the coldest month is January, when x = 1, and the hottest month is July, when x = 7. So choose d to be about 4.

The table shows that temperatures are actually a little warmer after July than before, so experiment with values just greater than 4 to find d.

Trial and error using a calculator leads to d = 4.2.


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(b) On the same coordinate axes, graph f for a two-year period together with the actual data values found in the table.

The graph shows the data points from the table, the graph

of

and the graph of

for

comparison.


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(c) Use the sine regression featrue of a graphing calculator to determine a second model for these data.


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Graphs of the Tangent and Cotangent Functions

4.3

Graph of the Tangent Function ▪ Graph of the Cotangent Function ▪ Graphing Techniques


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Graph of the Tangent Function

A vertical asymptote is a vertical line that the graph approaches but does not intersect, while function values increase or decrease without bound as x-values get closer and closer to the line.


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Tangent Function f(x) = tan x


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Tangent Function f(x) = tan x

The graph is discontinuous at values of x of the

form and has vertical asymptotes at these values.

Its x-intercepts are of the form x = nπ.

Its period is π.

Its graph has no amplitude, since there are no minimum or maximum values.

The graph is symmetric with respect to the origin, so the function is an odd function. For all x in the domain, tan(–x) = –tan(x).


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Cotangent Function f(x) = cot x


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The graph is discontinuous at values of x of the form x = nπ and has vertical asymptotes at these values.

Its x-intercepts are of the form .

Its period is π.


The graph is symmetric with respect to the origin, so the function is an odd function. For all x in the domain, cot(–x) = –cot(x).


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Tangent and Cotangent Functions

To graph the cotangent function, we must use one of the identities

The tangent function can be graphed directly with a graphing calculator using the tangent key.

since graphing calculators generally do not have cotangent keys.


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Guidelines for Sketching Graphs of Tangent and Cotangent Functions

Step 2 Sketch the two vertical asymptotes found in Step 1.

Step 1 Determine the period, To locate two adjacent vertical asymptotes, solve the following equations for x:

Step 3 Divide the interval formed by the vertical asymptotes into four equal parts.


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Guidelines for Sketching Graphs of Tangent and Cotangent Functions

Step 4 Evaluate the function for the first-quarter point, midpoint, and third-quarter point, using the x-values found in Step 3.

Step 5 Join the points with a smooth curve, approaching the vertical asymptotes. Indicate additional asymptotes and periods of the graph as necessary.


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Example 1 GRAPHING y = tan bx

Graph y = tan 2x.

Step 1 The period of this function is To locate two adjacent vertical asymptotes, solve

The asymptotes have equations and


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Example 1 GRAPHING y = tan bx (continued)

Step 2 Sketch the two vertical asymptotes.


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first-quarter value:

middle value: 0

third-quarter value:

Step 4 Evaluate the function for the x-values found in Step 3.


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Step 5 Join these points with a smooth curve, approaching the vertical asymptotes.

Draw another period by adding one-half period to the left and one-half period to the right.


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Example 2 GRAPHING y = a tan bx

Evaluate the function for the x-values found in Step 3 to obtain the key points

The period is To locate two adjacent vertical asymptotes, solve 2x = 0 and 2x = π to obtain x = 0 and

Divide the interval into four equal parts to obtain

the key x-values of


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Example 2 GRAPHING y = a tan bx (continued)

Plot the asymptotes and the points found in step 4. Join them with a smooth curve.

Because the coefficient –3 is negative, the graph is reflected across the x-axis compared to the graph of


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Note

The function defined byhas a graph that compares to the graph of y = tan x as follows:

The period is larger because

The graph is “stretched” because a = –3, and |–3| > 1.


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Each branch of the graph goes down from left to right (the function decreases) between each pair of adjacent asymptotes because a = –3, and –3 < 0.

When a < 0, the graph is reflected across the x-axis compared to the graph of y = |a| tan bx.


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Example 3 GRAPHING y = a cot bx

The period is Adjacent vertical asymptotes are

at x = –π and x = –π.

Divide the interval (–π, π) into four equal parts to obtain the key x-values of

Evaluate the function for the x-values found in Step 3 to obtain the key points


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Example 3 GRAPHING y = a cot bx (continued)

Plot the asymptotes and the points found in step 4. Join them with a smooth curve.


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Example 4 GRAPHING A TANGENT FUNCTION WITH A VERTICAL TRANSLATION

Graph y = 2 + tan x.

Every y value for this function will be 2 units more than the corresponding y value in y = tan x, causing the graph to be translated 2 units up compared to y = tan x.


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Example 4 GRAPHING A TANGENT FUNCTION WITH A VERTICAL TRANSLATION (cont.)

To see the vertical translation, observe the coordinates displayed at the bottoms of the screens.


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Example 5 GRAPHING A COTANGENT FUNCTION WITH VERTICAL AND HORIZONTAL TRANSLATIONS

The period is π because b = 1.

The graph will be translated down two units because c = –2.

The graph will be reflected across the x-axis because a = –1.

The phase shift is units to the right.


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Example 5 GRAPHING A COTANGENT FUNCTION WITH VERTICAL AND HORIZONTAL TRANSLATIONS (continued)

To locate adjacent asymptotes, solve

Divide the interval into four equal parts to

obtain the key x-values

Evaluate the function for the key x-values to obtain the key points


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Example 5 GRAPHING A COTANGENT FUNCTION WITH VERTICAL AND HORIZONTAL TRANSLATIONS (continued)

Plot the asymptotes and key points, then join them with a smooth curve.

An additional period to the left has been graphed.


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Graphs of the Secant and Cosecant Functions

4.4

Graph of the Secant Function ▪ Graph of the Cosecant Function ▪ Graphing Techniques ▪ Addition of Ordinates ▪ Connecting Graphs with Equations


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Secant Function f(x) = sec x


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The graph is discontinuous at values of x of the

form and has vertical asymptotes at these values.

There are no x-intercepts.

Its period is 2π.


The graph is symmetric with respect to the y-axis, so the function is an even function. For all x in the domain, sec(–x) = sec(x).


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Cosecant Function f(x) = csc x


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Cosecant Function f(x) = csc x


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The graph is discontinuous at values of x of the form x = nπ and has vertical asymptotes at these values.

There are no x-intercepts.

Its period is 2π.


The graph is symmetric with respect to the origin, so the function is an odd function. For all x in the domain, csc(–x) = –csc(x).


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Secant and Cosecant Functions

To graph the cosecant function on a graphing calculator, use the identity

To graph the secant function on a graphing calculator, use the identity


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Secant and Cosecant Functions


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Guidelines for Sketching Graphs of Cosecant and Secant Functions

Step 2 Sketch the vertical asymptotes. They will have equations of the form x = k, where k is an x-intercept of the graph of the guide function.

Step 1 Graph the corresponding reciprocal function as a guide, use a dashed curve.

y = cos bxy = a sec bx

y = a sin bxy = a csc bx

Use as a GuideTo Graph


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Guidelines for Sketching Graphs of Cosecant and Secant Functions

Step 3 Sketch the graph of the desired function by drawing the typical U-shaped branches between the adjacent asymptotes.

The branches will be above the graph of the guide function when the guide function values are positive and below the graph of the guide function when the guide function values are negative.


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Example 1 GRAPHING y = a sec bx

Step 1 Graph the corresponding reciprocal function

The function has amplitude 2 and one period of the graph lies along the interval that satisfies the inequality

Divide the interval into four equal parts and determine the key points.


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Example 1 GRAPHING y = a sec bx (continued)

Step 2 Sketch the vertical asymptotes. These occur at x-values for which the guide function equals 0, such as x = 3π, x = 3π, x = π, x = 3π.


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Example 1 GRAPHING y = a sec bx (continued)

Step 3 Sketch the graph of by drawing the typical U-shaped branches, approaching the asymptotes.


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Example 2 GRAPHING y = a csc (x – d)

Step 1 Graph the corresponding reciprocal function


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Example 2 GRAPHING y = a csc (x – d) (continued)

Step 2 Sketch the vertical asymptotes through the

x-intercepts of These have

the form where n is any integer.


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Example 2 GRAPHING y = a csc (x – d) (continued)

Step 3 Sketch the graph of by

drawing typical U-shaped branches between

the asymptotes.


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Addition of Ordinates

New functions can be formed by adding or subtracting other functions.

y = cos x + sin x

Graph y = cos x and y = sin x. Then, for selected values of x, plot the points (x, cos x + sin x) and join the points with a sinusoidal curve.


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Example 3(a) DETERMINING AN EQUATION FOR A GRAPH

Determine an equation for the graph.

This is the graph of y = tan x reflected across the y-axis and stretched vertically by a factor of 2.

An equation for the graph isy = –2 tan x.


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Example 3(b) DETERMINING AN EQUATION FOR A GRAPH


An equation for the graph isy = cot 2x.

This is the graph of y = cot x with period Therefore, b = 2.


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Example 3(c) DETERMINING AN EQUATION FOR A GRAPH


An equation for the graph isy = 1 + sec x.

This is the graph of y = sec x, translated one unit upward.


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Harmonic Motion4.5Simple Harmonic Motion ▪ Damped Oscillatory Motion


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Simple Harmonic Motion

The position of a point oscillating about an equilibrium position at time t is called simple harmonic motion.


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Simple Harmonic Motion

The position of a point oscillating about an equilibrium position at time t is modeled by either

where a and are constants, with

The amplitude of the motion is |a|, the period is and the frequency isoscillations per time unit.


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Example 1 MODELING THE MOTION OF A SPRING

Suppose that an object is attached to a coiled spring. It is pulled down a distance of 5 in. from its equilibrium position and then released. The time for one complete oscillation is 4 seconds.

(a) Give an equation that models the position of the object at time t.

When the object is released at t = 0, the object is at distance −4 in. from equilibrium.


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Example 1 MODELING THE MOTION OF A SPRING (continued)

Since the time needed to complete one oscillation is 4 sec, P = 4, so

The motion is modeled by


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Example 1 MODELING THE MOTION OF A SPRING (continued)

(b) Determine the position at t = 1.5 sec.

At t = 1.5 seconds, the object is about 3.54 in. above the equilibrium position.

(c) Find the frequency.

The frequency is the reciprocal of the period.


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Example 2 ANALYZING HARMONIC MOTION

Suppose that an object oscillates according to the model s(t) = 8 sin 3t, where t is in seconds and s(t) is in feet. Analyze the motion.

a = 8, so the object oscillates 8 feet in either direction from the starting point.

The motion is harmonic because the model is of the form

= time (in seconds) for one

complete oscillation


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Damped Oscillatory Motion

Most oscillatory motions are damped by the effect of friction which causes the amplitude of the motion to diminish gradually until the weight comes to rest.

This motion can be modeled by


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Damped Oscillatory Motion

copyright © 2009 pearson addison-wesley 4.1-1 4 graphs of the circular functions

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