E. Applications of Rational Functions
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CHAPTER 4 Polynomial and Rational Functions
4.4 EXERCISES
ᮣ
CONCEPTS AND VOCABULARY
Fill in each blank with the appropriate word or phrase. Carefully reread the section if needed.
1. Write the following in direction/approach notation.
As x becomes an infinitely large negative number, y
approaches 2.
2. For any constant k, the notation as ͿxͿ S ϩq,
y S k is an indication of a
asymptote,
while x S k, ͿyͿ S ϩq indicates a
asymptote.
3. Vertical asymptotes are found by setting the
equal to zero. The x-intercepts are found
by setting the
equal to zero.
4. If the degree of the numerator is equal to the
degree of the denominator, a horizontal asymptote
occurs at y ϭ ab , where ab represents the ratio of
the
.
3x2 Ϫ 2x
and a table of
2x2 Ϫ 3
values to discuss the concept of horizontal
asymptotes. At what positive value of x is the graph
of g within 0.01 of its horizontal asymptote?
5. Use the function g1x2 ϭ
ᮣ
6. Name all of the “tools” at your disposal that play a
role in the graphing of rational functions. Which
tools are indispensable and always used? Which are
used only as the situation merits?
DEVELOPING YOUR SKILLS
Give the location of the vertical asymptote(s) if they
exist, and state the function’s domain.
7. f 1x2 ϭ
xϩ2
xϪ3
9. g1x2 ϭ
3x2
x2 Ϫ 9
8. F1x2 ϭ
4x
2x Ϫ 3
10. G1x2 ϭ
xϩ1
9x2 Ϫ 4
11. h1x2 ϭ
x2 Ϫ 1
xϪ5
12. H1x2 ϭ 2
2x ϩ 3x Ϫ 5
2x Ϫ x Ϫ 3
13. p1x2 ϭ
2x ϩ 3
2
x ϩxϩ1
2
14. q1x2 ϭ
2x3
x2 ϩ 4
Give the location of the vertical asymptote(s) if they
exist, and state whether function values will change sign
(positive to negative or negative to positive) from one
side of the asymptote to the other.
15. Y1 ϭ
xϩ1
x ϪxϪ6
2
17. r1x2 ϭ
16. Y2 ϭ
2x ϩ 3
x Ϫ x Ϫ 20
2
x2 ϩ 3x Ϫ 10
x2 Ϫ 2x Ϫ 15
18.
R1x2
ϭ
x2 Ϫ 6x ϩ 9
x2 Ϫ 4x ϩ 4
19. Y1 ϭ
x
x ϩ 2x Ϫ 4x Ϫ 8
20. Y2 ϭ
Ϫ2x
x3 ϩ x2 Ϫ x Ϫ 1
3
2
For the functions given, (a) determine if a horizontal
asymptote exists and (b) determine if the graph will
cross the asymptote, and if so, where it crosses.
21. Y1 ϭ
2x Ϫ 3
x2 ϩ 1
22. Y2 ϭ
4x ϩ 3
2x2 ϩ 5
23. r1x2 ϭ
4x2 Ϫ 9
2x2 Ϫ x Ϫ 10
24.
R1x2
ϭ
x2 Ϫ 3x Ϫ 18
x2 ϩ 5
25. p1x2 ϭ
3x2 Ϫ 5
x2 Ϫ 1
26. P1x2 ϭ
3x2 Ϫ 5x Ϫ 2
x2 Ϫ 4
Apply long division to find the quotient and remainder
for each function. Use this information to determine the
equation of the horizontal asymptote, and whether the
graph will cross this asymptote. Verify answers by
graphing the functions on a graphing calculator and
locating points of intersection.
27. v1x2 ϭ
8x
x ϩ1
28. f 1x2 ϭ
4x ϩ 8
x2 ϩ 1
29. g1x2 ϭ
2x2 Ϫ 8x
x2 Ϫ 4
30. h1x2 ϭ
x2 Ϫ x Ϫ 6
x2 Ϫ 1
2
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Give the location of the x- and y-intercepts (if they
exist), and discuss the behavior of the function (bounce
or cross) at each x-intercept.
x2 Ϫ 3x
31. f 1x2 ϭ 2
x Ϫ5
2x Ϫ x2
32. F1x2 ϭ 2
x ϩ 2x Ϫ 3
33. g1x2 ϭ
x2 ϩ 3x Ϫ 4
x2 ϩ 7x ϩ 6
34.
G1x2
ϭ
x2 Ϫ 1
x2 Ϫ 2
35. h1x2 ϭ
x3 Ϫ 6x2 ϩ 9x
4x ϩ 4x2 ϩ x3
36. H1x2 ϭ
2
4Ϫx
x2 Ϫ 1
Use the Guidelines for Graphing Rational Functions to
graph the functions given.
xϩ3
37. f 1x2 ϭ
xϪ1
39. F1x2 ϭ
8x
x ϩ4
2
Ϫ2x
41. p1x2 ϭ 2
x Ϫ4
2
40. G1x2 ϭ
Ϫ12x
x2 ϩ 3
Ϫ3x
2x
46. H1x2 ϭ 2
x2 Ϫ 6x ϩ 9
x Ϫ 2x ϩ 1
xϪ1
x Ϫ 3x Ϫ 4
49. s1x2 ϭ
4x2
2x2 ϩ 4
x2 Ϫ 4
51. Y1 ϭ 2
x Ϫ1
54. V1x2 ϭ
3x
x ϩx ϪxϪ1
48. Y2 ϭ
1Ϫx
x2 Ϫ 2x
50. S1x2 ϭ
Ϫ2x2
x2 ϩ 1
x2 Ϫ x Ϫ 12
52. Y2 ϭ 2
x ϩ x Ϫ 12
3
3
2
Use the vertical asymptotes, x-intercepts, and their
multiplicities to construct an equation that corresponds
to each graph. Be sure the y-intercept estimated from
the graph matches the value given by your equation for
x ؍0. Check work on a graphing calculator.
55.
56.
y
10
8
6
4
2
Ϫ10Ϫ8Ϫ6Ϫ4Ϫ2
Ϫ2
57.
2 4 6 8 10 x
Ϫ4
Ϫ6
Ϫ8
Ϫ10
58.
y
Ϫ4
Ϫ6
Ϫ8
Ϫ10
10
8
6
4
2
Ϫ10Ϫ8Ϫ6Ϫ4Ϫ2
Ϫ2
2 4 6 8 10 x
10
8
6
4
2
Ϫ10Ϫ8Ϫ6Ϫ4Ϫ2
Ϫ2
y
(Ϫ2, 2)
(1, 1)
Ϫ4
Ϫ6
Ϫ8
Ϫ10
3x
42. P1x2 ϭ 2
x Ϫ9
45. h1x2 ϭ
2
Ϫ2x
x ϩ 2x2 Ϫ 4x Ϫ 8
2
2x Ϫ x2
x2 ϩ 3x
44.
Q1x2
ϭ
x2 ϩ 4x Ϫ 5
x2 Ϫ 2x Ϫ 3
47. Y1 ϭ
53. v1x2 ϭ
xϪ4
38. g1x2 ϭ
xϩ2
43. q1x2 ϭ
ᮣ
441
Section 4.4 Graphing Rational Functions
4 6 8 10 x
5, Ϫ Ò
16
y
10
8
6
4
2
Ϫ10Ϫ8Ϫ6Ϫ4Ϫ2
Ϫ2
(3, 4)
2 4 6 8 10 x
Ϫ4
Ϫ6
Ϫ8
Ϫ10
Graph the following using the Guidelines for Graphing
Rational Functions. From the equations given, note
there are no horizontal asymptotes (the degree of each
numerator is greater than the degree of the
denominator) and a large number of plotted points may
be necessary to complete each graph.
59. v1x2 ϭ
x2 Ϫ 4
x
60. f 1x2 ϭ
9 Ϫ x2
xϪ1
61. g1x2 ϭ
x2
xϪ1
62. h1x2 ϭ
1 Ϫ x2
xϩ2
WORKING WITH FORMULAS
ax
x ؉b
The population density of urban areas (in people
per square mile) can be modeled by the formula
shown, where a and b are constants related to the
overall population and sprawl of the area under
study, and D(x) is the population density (in
hundreds), x mi from the center of downtown.
63. Population density: D1x2 ؍
2
Graph the function for a ϭ 63 and b ϭ 20 over the
interval x ʦ 3 0, 504 , and then use the graph to
answer the following questions.
a. What is the significance of the horizontal
asymptote (what does it mean in this context)?
b. How far from downtown does the population
density fall below 525 people per square mile?
How far until the density falls below 300
people per square mile?
c. Use the graph and a table to determine how far
from downtown the population density reaches
a maximum. What is this maximum?
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kx
100 ؊ x
Some industries resist cleaner air standards because
the cost of removing pollutants rises dramatically as
higher standards are set. This phenomenon can be
modeled by the formula given, where C(x) is the
cost (in thousands of dollars) of removing x% of the
pollutant and k is a constant that depends on the type
of pollutant and other factors.
Graph the function for k ϭ 250 over the interval
x ʦ 3 0, 1004 , and then use the graph to answer the
following questions.
4–62
64. Cost of removing pollutants: C1x2 ؍
ᮣ
a. What is the significance of the vertical
asymptote (what does it mean in this context)?
b. If new laws are passed that require 80% of a
pollutant to be removed, while the existing law
requires only 75%, how much will the new
legislation cost the company? Compare the
cost of the 5% increase from 75% to 80% with
the cost of the 1% increase from 90% to 91%.
c. What percent of the pollutants can be removed
if the company budgets 2250 thousand dollars?
APPLICATIONS
C(h)
65. Medication in the blood0.4
stream: The concentration C 0.3
0.2
of a certain medicine in the
0.1
bloodstream h hours after
2 6 10 14 18 22 26 h
being injected into the
shoulder is given by the
2h2 ϩ h
. Use the given graph of
function: C1h2 ϭ 3
h ϩ 70
the function to answer the following questions.
a. Approximately how many hours after injection
did the maximum concentration occur? What
was the maximum concentration?
b. Use C(h) to compute the rate of change for the
intervals h ϭ 8 to h ϭ 10 and h ϭ 20 to
h ϭ 22. What do you notice?
c. Use mathematical notation to state what
happens to the concentration C as the number of
hours becomes infinitely large. What role does
the h-axis play for this function?
66. Supply and demand: In
S(t)
10.0
response to certain market
7.5
demands, manufacturers will 5.0
2.5
quickly get a product out on
the market to take advantage
10 20 30 40 50 60 70 t
of consumer interest. Once
the product is released, it is not uncommon for
sales to initially skyrocket, taper off, and then
gradually decrease as consumer interest wanes. For
a certain product, sales can be modeled by the
250t
, where S(t) represents the
function S1t2 ϭ 2
t ϩ 150
daily sales (in $10,000) t days after the product has
debuted. Use the given graph of the function to
answer the following questions.
a. Approximately how many days after the
product came out did sales reach a maximum?
What was the maximum sales?
b. Use S(t) to compute the rate of change for the
intervals t ϭ 7 to t ϭ 8 and t ϭ 60 to t ϭ 62.
What do you notice?
c. Use mathematical notation to state what
happens to the daily sales S as the number of
days becomes infinitely large. What role does
the t-axis play for this function?
67. Cost to remove pollutants: For a certain coalburning power plant, the cost to remove pollutants
from plant emissions can be modeled by
80p
C1p2 ϭ
, where C(p) represents the cost
100 Ϫ p
(in thousands of dollars) to remove p percent of the
pollutants. (a) Find the cost to remove 20%, 50%,
and 80% of the pollutants, then comment on the
results; (b) graph the function using an appropriate
scale; and (c) use mathematical notation to state
what happens if the power company attempts to
remove 100% of the pollutants.
68. Costs of recycling: A large city has initiated a new
recycling effort, and wants to distribute recycling bins
for use in separating various recyclable materials.
City planners anticipate the cost of the program can
220p
, where
be modeled by the function C1p2 ϭ
100 Ϫ p
C(p) represents the cost (in $10,000) to distribute
the bins to p percent of the population. (a) Find the
cost to distribute bins to 25%, 50%, and 75% of the
population, then comment on the results; (b) graph
the function using an appropriate scale; and (c) use
mathematical notation to state what happens if the
city attempts to give recycling bins to 100% of the
population.
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Memory retention: Due to their asymptotic behavior,
rational functions are often used to model the mind’s ability
to retain information over a long period of time — the “use
it or lose it” phenomenon.
69. Language retention: A large group of students is
asked to memorize a list of 50 Italian words, a
language that is unfamiliar to them. The group is
then tested regularly to see how many of the words
are retained over a period of time. The average
number of words retained is modeled by the
6t ϩ 40
function W1t2 ϭ
, where W(t) represents
t
the number of words remembered after t days.
a. Graph the function over the interval t ʦ 3 1, 404 .
How many days until only half the words are
remembered? How many days until only onefifth of the words are remembered?
b. After 10 days, what is the average number of
words retained? How many days until only
8 words can be recalled?
c. What is the significance of the horizontal
asymptote (what does it mean in this
context)?
70. Language retention: A similar study asked
students to memorize 50 Hawaiian words, a
language that is both unfamiliar and phonetically
foreign to them (see Exercise 69). The average
number of words retained is modeled by the
4t ϩ 20
, where W(t) represents
function W1t2 ϭ
t
the number of words after t days.
a. Graph the function over the interval t ʦ 31, 40 4.
How many days until only half the words are
remembered? How does this compare to
Exercise 69? How many days until only onefifth of the words are remembered?
b. After 7 days, what is the average number of
words retained? How many days until only
5 words can be recalled?
c. What is the significance of the horizontal
asymptote (what does it mean in this
context)?
Concentration and dilution: When
antifreeze is mixed with water, it
becomes diluted — less than 100%
antifreeze. The more water added,
the less concentrated the antifreeze
becomes, with this process continuing until a desired
concentration is met. This application and many similar to
it can be modeled by rational functions.
Section 4.4 Graphing Rational Functions
443
71. Concentration of antifreeze: A 400-gal tank
currently holds 40 gal of a 25% antifreeze
solution. To raise the concentration of the
antifreeze in the tank, x gal of a 75% antifreeze
solution is pumped in.
a. Show the formula for the resulting
40 ϩ 3x
concentration is C1x2 ϭ
after
160 ϩ 4x
simplifying, and graph the function over the
interval x ʦ 30, 360 4 .
b. What is the concentration of the antifreeze in the
tank after 10 gal of the new solution are added?
After 120 gal have been added? How much
liquid is now in the tank?
c. If the concentration level is now at 65%, how
many gallons of the 75% solution have been
added? How many gallons of liquid are in the
tank now?
d. What is the maximum antifreeze concentration
that can be attained in a tank of this size? What
is the maximum concentration that can be
attained in a tank of “unlimited” size?
72. Concentration of sodium chloride: A sodium
chloride solution has a concentration of 0.2 oz
(weight) per gallon. The solution is pumped into an
800-gal tank currently holding 40 gal of pure
water, at a rate of 10 gal/min.
a. Find a function A(t) modeling the amount of
liquid in the tank after t min, and a function
S(t) for the amount of sodium chloride in the
tank after t min.
b. The concentration C(t) in ounces per gallon is
S1t2
, a rational function.
measured by the ratio
A1t2
Graph the function on the interval t ʦ 30, 1004.
What is the concentration level (in ounces per
gallon) after 6 min? After 28 min? How many
gallons of liquid are in the tank at this time?
c. If the concentration level is now 0.184 oz/gal,
how long have the pumps been running?
How many gallons of liquid are in the tank
now?
d. What is the maximum concentration that can
be attained in a tank of this size? What is the
maximum concentration that can be attained in
a tank of “unlimited” size?
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Average cost of manufacturing an item: The cost “C” to
manufacture an item depends on the relatively fixed costs
“K” for remaining in business (utilities, maintenance,
transportation, etc.) and the actual cost “c” of
manufacturing the item (labor and materials). For x items
the cost is C1x2 ϭ K ϩ cx. The average cost
C1x2
“A” of manufacturing an item is then A1x2 ϭ
.
x
73. Manufacturing water heaters: A company that
manufactures water heaters finds their fixed costs
are normally $50,000 per month, while the cost to
manufacture each heater is $125. Due to factory
size and the current equipment, the company can
produce a maximum of 5000 water heaters per
month during a good month.
a. Use the average cost function to find the
average cost if 500 water heaters are
manufactured each month. What is the average
cost if 1000 heaters are made?
b. What level of production will bring the average
cost down to $150 per water heater?
c. If the average cost is currently $137.50, how
many water heaters are being produced that
month?
d. What’s the significance of the horizontal
asymptote for the average cost function (what
does it mean in this context)? Will the
company ever break the $130 average cost
level? Why or why not?
74. Producing biodegradable disposable diapers:
An enterprising company has finally developed a
better disposable diaper that is biodegradable. The
brand becomes wildly popular and production is
soaring. The fixed cost of production is $20,000
per month, while the cost of manufacturing is
$6.00 per case (48 diapers). Even while working
three shifts around-the-clock, the maximum
production level is 16,000 cases per month. The
company figures it will be profitable if it can bring
costs down to an average of $7 per case.
a. Use the average cost function to find the
average cost if 2000 cases are produced each
month. What is the average cost if 4000 cases
are made?
b. What level of production will bring the average
cost down to $8 per case?
c. If the average cost is currently $10 per case,
how many cases are being produced?
d. What’s the significance of the horizontal
asymptote for the average cost function (what
does it mean in this context)? Will the
company ever reach its goal of $7/case at its
maximum production? What level of
production would help them meet their goal?
4–64
Test averages and grade point
averages: To calculate a test
average we sum all test points P
and divide by the number of
P
tests N: . To compute the
N
score or scores needed on future tests to raise the average
grade to a desired grade G, we add the number of additional
tests n to the denominator, and the number of additional
tests times the projected grade g on each test to the
numerator:
P ϩ ng
G1n2 ϭ
. The result is a rational function with
Nϩn
some “eye-opening” results.
75. Computing an average grade: After four tests,
Bobby Lou’s test average was an 84.
[Hint: P ϭ 41842 ϭ 336.]
a. Assume that she gets a 95 on all remaining
tests 1g ϭ 952. Graph the resulting function on
a calculator using the window n ʦ 30, 20 4 and
G1n2 ʦ 380 to 100 4 . Use the calculator to
determine how many tests are required to lift
her grade to a 90 under these conditions.
b. At some colleges, the range for an “A” grade is
93–100. How many tests would Bobby Lou
have to score a 95 on, to raise her average to
higher than 93? Were you surprised?
c. Describe the significance of the horizontal
asymptote of the average grade function. Is a
test average of 95 possible for her under these
conditions?
d. Assume now that Bobby Lou scores 100 on all
remaining tests 1g ϭ 1002. Approximately how
many more tests are required to lift her grade
average to higher than 93?
76. Computing a GPA: At most colleges, A S 4
grade points, B S 3, C S 2, and D S 1. After
taking 56 credit hours, Aurelio’s GPA is 2.5.
[Hint: In the formula given, P ϭ 2.51562 ϭ 140.]
a. Assume Aurelio is determined to get A’s
(4 grade points or g ϭ 42, for all remaining
credit hours. Graph the resulting function on a
calculator using the window n ʦ 3 0, 604 and
G1n2 ʦ 3 2, 4 4. Use the calculator to determine
the number of credit hours required to lift his
GPA to over 2.75 under these conditions.
b. At some colleges, scholarship money is
available only to students with a 3.0 average or
higher. How many (perfect 4.0) credit hours
would Aurelio have to earn, to raise his GPA to
3.0 or higher? Were you surprised?
c. Describe the significance of the horizontal
asymptote of the GPA function. Is a GPA of
4.0 possible for him under these conditions?
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ᮣ
Section 4.5 Additional Insights into Rational Functions
EXTENDING THE CONCEPT
77. In addition to determining if a function has a
vertical asymptote, we are often interested in how
fast the graph approaches the asymptote. As in
previous investigations, this involves the function’s
rate of change over a small interval. Exercise 64
describes the rising cost of removing pollutants
from the air. As noted there, the rate of increase in
the cost changes as higher requirements are set. To
quantify this change, we’ll compute the rate of
C1x2 2 Ϫ C1x1 2
¢C
250x
ϭ
.
change
for C1x2 ϭ
x2 Ϫ x1
¢x
100 Ϫ x
a. Find the rate of change of the function in the
following intervals:
x ʦ 3 60, 614 x ʦ 3 70, 71 4
x ʦ 3 80, 814 x ʦ 390, 91 4
b. What do you notice? How much did the rate
increase from the first interval to the second?
From the second to the third? From the third to
the fourth?
ᮣ
445
c. Recompute parts (a) and (b) using the function
350x
C1x2 ϭ
. Comment on what you notice.
100 Ϫ x
ax2 ϩ k
, where a, b,
bx2 ϩ h
k, and h are constants and a, b 7 0.
a. What can you say about asymptotes and
intercepts of this function if h, k 7 0?
b. Now assume k 6 0 and h 7 0. How does this
affect the asymptotes? The intercepts?
c. If b ϭ 1 and a 7 1, how does this affect the
results from part (b)?
d. How is the graph affected if k 7 0 and h 6 0?
e. Find values of a, b, h, and k that create a
function with a horizontal asymptote at y ϭ 32,
x-intercepts at 1Ϫ2, 02 and (2, 0), a y-intercept
of 10, Ϫ42 , and no vertical asymptotes.
78. Consider the function f 1x2 ϭ
MAINTAINING YOUR SKILLS
79. (1.4) Find the equation of a line that is
perpendicular to 3x Ϫ 4y ϭ 12 and contains the
point 12, Ϫ32.
80. (4.1) Use synthetic division and the remainder
theorem to find the value of f (4), f 1 32 2 , and
f (2):f 1x2 ϭ 2x3 Ϫ 7x2 ϩ 5x ϩ 3.
81. (3.2) Solve the following equation using the
quadratic formula, then write the equation in
factored form: 12x2 ϩ 55x Ϫ 48 ϭ 0.
82. (R.1/3.1) Describe/Define each set of numbers:
complex ރ, rational ޑ, and integers ޚ.
4.5
Additional Insights into Rational Functions
LEARNING OBJECTIVES
In Section 4.5 you will see
how we can:
A. Graph rational functions with
removable discontinuities
B. Graph rational functions
with oblique or nonlinear
asymptotes
C. Solve applications involving
rational functions
In Section 4.4, we studied rational functions whose graphs had horizontal and/or vertical asymptotes. In this section, we’ll study functions with asymptotes that are neither
horizontal nor vertical. In addition, we’ll further explore the “break” we saw in graphs
of certain piecewise-defined functions, that of a simple “hole” created when the numerator and denominator of a rational function share a common variable factor.
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CHAPTER 4 Polynomial and Rational Functions
A. Rational Functions and Removable Discontinuities
In Example 6 of Section 2.5, we graphed the piecewise-defined function
x2 Ϫ 4
x 2
h1x2 ϭ • x Ϫ 2
. The second piece is simply the point (2, 1). The first piece
1
xϭ2
is a rational function, but instead of a vertical asymptote at x ϭ 2 (the zero of the
denominator), its graph is actually the line y ϭ x ϩ 2 with a “hole” at (2, 4) called a
removable discontinuity (Figure 4.60). The hole occurs because the numerator and
x2 Ϫ 4
denominator of y ϭ
share the common factor 1x Ϫ 22 , and canceling these
xϪ2
factors leaves y ϭ x ϩ 2, a continuous function. However, the original function is not
defined at x ϭ 2, so we must delete the single point at x ϭ 2, y ϭ 4 from the graph of
the line (Figure 4.60).
Figure 4.60
Figure 4.61
y
y
5
5
4
4
(2, 4)
3
2
h(x)
2
H(x)
(2, 1)
1
Ϫ5 Ϫ4 Ϫ3 Ϫ2 Ϫ1
Ϫ1
(2, 4)
3
1
2
3
4
5
1
Ϫ5 Ϫ4 Ϫ3 Ϫ2 Ϫ1
Ϫ1
x
Ϫ2
Ϫ2
Ϫ3
Ϫ3
Ϫ4
Ϫ4
Ϫ5
Ϫ5
1
2
3
4
5
x
We can remove or fix this discontinuity by redefining the second piece
as h1x2 ϭ 4, when x ϭ 2. This would create a new and continuous function,
x2 Ϫ 4
x 2
H1x2 ϭ • x Ϫ 2
(Figure 4.61).
4
xϭ2
It’s possible for a rational graph to have more than one removable discontinuity, or
to be nonlinear with a removable discontinuity. For cases where we elect to repair the
break, we will adopt the convention of using the corresponding upper case letter to
name the new function, as we did here.
EXAMPLE 1
ᮣ
Graphing Rational Functions with Removable Discontinuities
x3 ϩ 8
. If there is a removable discontinuity, repair the
xϩ2
break using an appropriate piecewise-defined function.
Graph the function t1x2 ϭ
Solution
ᮣ
Note the domain of t does not include x ϭ Ϫ2. We begin by factoring as before to
identify zeroes and asymptotes, but find the numerator and denominator share a
common factor, which we remove.
t1x2 ϭ
ϭ
x3 ϩ 8
xϩ2
1x ϩ 221x2 Ϫ 2x ϩ 42
xϩ2
ϭ x Ϫ 2x ϩ 4; where x
2
Ϫ2
The graph of t will be the same as y ϭ x Ϫ 2x ϩ 4 for all values except x ϭ Ϫ2.
Here we have a parabola, opening upward, with y-intercept (0, 4). From the vertex
Ϫ1Ϫ22
Ϫb
ϭ 1, giving y ϭ 3
formula, the x-coordinate of the vertex will be
ϭ
2a
2112
2
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College Algebra Graphs & Models—
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Section 4.5 Additional Insights into Rational Functions
after substitution. The vertex is (1, 3). Evaluating
t1Ϫ12 we find 1Ϫ1, 72 is on the graph, giving the
point (3, 7) using the axis of symmetry. We draw
a parabola through these points, noting the
original function is not defined at Ϫ2, and there
will be a “hole” in the graph at 1Ϫ2, y2. The
value of y is found by substituting Ϫ2 for x in the
simplified form: 1Ϫ22 2 Ϫ 21Ϫ22 ϩ 4 ϭ 12. This
information produces the graph shown. We can
repair the break using the function
x3 ϩ 8
x Ϫ2
T1x2 ϭ • x ϩ 2
12
x ϭ Ϫ2
t(x)
(4, 12)
from
symmetry
(Ϫ2, 12)
10
(3, 7)
from
symmetry
(Ϫ1, 7)
5
(1, 3)
5
x
axis of
symmetry
Now try Exercises 7 through 18
The
ᮣ
feature of a graphing calculator is a wonderful tool for understanding the
f 1x2
, where
characteristics of a function. We’ll illustrate using the function h1x2 ϭ
g1x2
TRACE
f 1x2 ϭ x3 Ϫ 2x2 Ϫ 3x ϩ 6 and g1x2 ϭ x Ϫ 2 (similar to Example 1). Enter
A. You’ve just seen how
we can graph rational
functions with removable
discontinuities
the Y= screen as Y1 (Figure 4.62), then graph
the function using ZOOM 4:ZDecimal. Recall
this will allow the calculator to trace through
¢x intervals of 0.1. After pressing the TRACE
key, the cursor appears on the graph at the
y-intercept 10, Ϫ32 and its location is displayed
at the bottom of the screen. Note that there is a
“hole” in the graph in the first quadrant (Figure 4.63). We can walk the cursor along the
curve in either direction using the left arrow
and right arrow
keys to determine exactly
where this hole occurs. Walking the cursor to
the right, we note that no output is displayed
for x ϭ 2.
f 1x2
Next, simplify
and enter the result
g1x2
as Y2. Factoring the numerator by grouping
and reducing the common factors gives
1x2 Ϫ 32 1x Ϫ 22
f 1x2
ϭ
, so Y2 ϭ x2 Ϫ 3.
g1x2
1x Ϫ 22
Graphing both functions reveals that they are
identical, except that Y2 ϭ x2 Ϫ 3 covers the
hole left by Y1 using x ϭ 2, y ϭ 1. In other
words, Y1 is equivalent to Y2 except at x ϭ 2.
This can also be seen using the TABLE feature
of a calculator, which displays an error message
for Y1 when x ϭ 2 is input, but shows an output
of 1 for Y2 (Figure 4.64). The bottom line is—
the domain of h is all real numbers except x ϭ 2.
f 1x2
g1x2
on
Figure 4.62
Figure 4.63
3.1
Ϫ4.7
4.7
Ϫ3.1
Figure 4.64