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3-4 Adding and Subtracting Rational Expressions

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4-4
Adding and
Subtracting Rational
Expressions
PearsonRealize.com
I CAN… find the sum
or difference of rational
expressions.
VOCABULARY
• compound fraction
ESSENTIAL QUESTION
Activity
CRITIQUE & EXPLAIN
Teo and Shannon find the following exercise in their homework:
__
​​  1 ​+ __
​  1 ​+ __
​  1 ​​
2
Compare addition of numerical
and algebraic fractions:
+3 _
_
​​  15 ​ + _​  35 ​ = ​  1___
 ​ = ​  45 ​​
5
In the same way,
+5
x
​​ ____
​​ + ____
​​  x +5 4 ​​ = ____
​​  xx +
​​.
x+4
4
3
9
A. Teo claims that a common denominator of
the sum is 2 + 3 + 9 = 14. Shannon claims that
it is 2 ∙ 3 ∙ 9 = 54. Is either student correct?
Explain why or why not.
1+1+1
2 3 9
B. Find the sum, explaining the method you use.
C. Construct Arguments Timothy states that
the quickest way to find the sum of any
two fractions with unlike denominators is
to multiply their denominators to find a
common denominator, and then rewrite each
fraction with that denominator. Do you agree?
How do you rewrite rational expressions to find sums and differences?
Add Rational Expressions With Like Denominators
EXAMPLE 1
USE STRUCTURE
Assess
What is the sum?
x ​ + _____
A. ​​ _____
​  5 ​​
x+4
x+4
x+5
= ​​ _____
​​
When denominators are the same, add the numerators.
x+4
So _____
​​  x ​ + _____
​  5 ​​ = _____
​​  x + 5 ​​
x+4
x+4
x+4
2x + 1
B. ​ _______
​+ ________
​​  3x − 8  ​​
2
​x​​  ​ + 3x
x(x + 3)
(2x + 1) + (3x − 8)
​x​​  ​ + 3x
________________
= ​​   
  ​​
2
Add the numerators.
(2x + 3x) + (1 − 8)
​x​​  ​ + 3x
________________
=   
​​ 
  ​​
2
Use the Commutative and Associative Properties.
− 7 ​​
= _______
​​  5x
2
Combine like terms.
x + 3x
+1
− 7 ​​
So _______
​​  2x
​​ + _______
​​  3x − 8 ​​ = _______
​​  5x
2
2
​x​​  ​ + 3x
x(x + 3)
​x​​  ​ + 3x
Try It! 1. Find the sum.
10x − 5 ​ + ______
a.​​ _______
​  8 − 4x ​​
2x + 3
2x + 3
x − 5 ​ + _______
b.​​ _____
​  3x − 21 ​​
x+5
x+5
LESSON 4-4 Adding and Subtracting Rational Expressions
217
Activity
CONCEPTUAL
UNDERSTANDING
Assess
Identify the Least Common Multiple of Polynomials
EXAMPLE 2
How can you find the least common multiple (LCM) of polynomials?
A. ​​(x + 2)​​  2​, ​x​​  2​ + 5x + 6​
B. ​​x​​  3​ − 9x, ​x​​  2​ − 2x − 15, ​x​​  2​ − 5x​
STUDY TIP
Factor each polynomial.
Factor each polynomial.
Using the LCM of the
denominators can mean less
work simplifying later.
​​(x + 2)​​  2​ = (x + 2)(x + 2)​
​​x​​  3​ − 9x = x(​x​​  2​ − 9) = x(x + 3)(x − 3)​
​​x​​  2​ + 5x + 6 = (x + 2)(x + 3)​
​​x​​  2​ − 2x − 15 = (x + 3)(x − 5)​
The LCM is the product of the
factors. Duplicate factors are
raised to the greatest power
represented.
​​x​​  2 ​− 5x = x(x − 5)​
​LCM: x(x + 3)(x − 3)(x − 5)​
Each original polynomial
is a factor of the LCM.
LCM: ​(x + 2)(x + 2)(x + 3)​ or
​​(x + 2)​​  2​(x + 3)​
Try It! 2. Find the LCM for each set of expressions.
a. ​​x​​  3​ + ​9x​​  2​ + 27x + 27, ​x​​  2​ − 4x − 21​
b. ​​10x​​  2​ − ​10y​​  2​, ​15x​​  2​ − 30xy + ​15y​​  2​, ​x​​  2​ + 3xy + ​2y​​  2​​
Add Rational Expressions With Unlike Denominators
EXAMPLE 3
What is the sum of​_____
​ x2+ 3 ​ ​and​ _________
​  2 2
​​?
​x​​  ​ − 1
USE STRUCTURE
The LCM of 6 and 15 is 30, not 90.
1
_
​​  16 ​​ + _
​​  15
 ​​ = ___
​​  2 1∙ 3 ​​ + ___
​​  3 1∙ 5 ​​ = ______
​​  1 ∙25∙ +3 ∙15∙ ​​2
​x​​  ​ − 3x + 2
Follow a similar procedure to the one you use to add numerical fractions
with unlike denominators.
x+3
2
2
______
​​  x + 3 ​ + __________
​ 
​ = ___________
​    
​+ ___________
​    
​
​ ​​  2​ − 1
x
(x + 1)(x − 1)
​ ​​  2​ − 3x + 2
x
(x + 3)(x − 2)
The LCM does not contain the
common factor 3 twice. In the
example problem, the common
factor (x − 1) is not used
twice.
Factor each
denominator.
(x − 1)(x − 2)
2(x + 1)
​= _________________
  
​    ​+ _________________
​   
  ​ Use the LCM as
(x + 1)(x − 1)(x − 2) (x + 1)(x − 1)(x − 2)
the least common
denominator (LCD).
(x + 3)(x − 2) + 2(x + 1)
(x + 1)(x − 1)(x − 2)
Add the numerators.
(​x​​  2​ + x − 6) + (2x + 2)
(x + 1)(x − 1)(x − 2)
Distribute.
​= ​ ____________________
  
  
 ​
​
___________________
​=   
​    ​
2
​x​​  ​ + 3x − 4
_________________
​= ​   
  ​
Combine like terms.
(x + 1)(x − 1)(x − 2)
(x + 4)(x − 1)
(x + 1)(x − 1)(x − 2)
​= _________________
  
​    ​​​​
(x + 4)
(x + 1)(x − 2)
Factor.
(x − 1)
(x − 1)
​=​ ___________
​​    ​​ ∙ ______
​​ 
​​
Rewrite to identify
unit factors.
x+4
​= ​ ___________
  
 ​ ​for x ≠ −1, 1, and 2
Simplify and
(x + 1)(x − 2)
state the domain.
x+3
x+4
2
_____
________
__________
The sum of ​​  2 ​​ and ​​  2
​​ is ​​ 
​​ for x ≠ −1, 1, and 2.
​x​​  ​ −1
​x​​  ​ −3x + 2
(x + 1)(x − 2)
Try It! 3. Find the sum.
x+6
a. ​​ ______
 ​ + __________
​  2 2
 ​​
2
​x​​  ​ − 4
218 TOPIC 4 Rational Functions
​x​​  ​ − 5x + 6
2
2x ​ +   
x​​  ​ − 11x − 12 ​​
_____________
b.​​ ______
​  ​4  
2
3x + 4
​6x​​  ​ + 5x − 4
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Activity
Assess
Subtract Rational Expressions
EXAMPLE 4
What is the difference between __________
​​  2 x + 1 ​​ and _________
​​  2 x + 1 ​​?
​x​​  ​ + 6x + 8
​x​​  ​ − 6x − 16
x+1
x+1
x+1
​​ ___________
  ​​ − __________
​​  2 x + 1 ​​ = ___________
​​    
​​ − ___________
​​    
​​
2
​x​​  ​ − 6x − 16
(x − 8)(x + 2)
​x​​  ​ + 6x + 8
(x + 2)(x + 4)
(x + 1)(x + 4)
(x − 8)(x + 2)(x + 4)
The LCD is
(x − 8)(x + 2)
(x + 4).
(x − 8)(x + 1)
(x − 8)(x + 2)(x + 4)
_________________
​= ​ _________________
  
  ​−   
​    
(​x​​  2​ + 5x + 4) − (​x​​  2​ − 7x − 8)
(x − 8)(x + 2)(x + 4)
________________________
​
=
​    
  
COMMON ERROR
When subtracting polynomials,
remember to distribute ​−​1 when
removing the parentheses.
Check for common
​x​​  2​ + 5x + 4 − ​x​​  2​ + 7x + 8
_____________________
​
=
​    
  
factors in the numerator
(x − 8)(x + 2)(x + 4)
and denominator and
12x + 12
= _________________
​
  
​    
simplify, if possible.
(x − 8)(x + 2)(x + 4)
12(x + 1)
(x − 8)(x + 2)(x + 4)
_________________
​
=
​   
   ​​
12(x + 1)
_______________
The difference between __________
​​  2 x + 1 ​​ and _________
​​  2 x + 1 ​​ is   
​​    ​​
(x − 8)(x + 2)(x + 4)
​x​​  ​ − 6x − 16
​x​​  ​ + 6x + 8
for x ≠ −4, −2, and 8.
Try It! 4. Simplify.
3x − 5 ​ − _____
b.​​ _______
​  2 ​​
2
1 ​+ ___
a.​​ ___
​  1 ​ − ___
​  12 ​
3x
APPLICATION
6x
​x​​  ​− 25
​x​​  ​
x+5
Find a Rate
EXAMPLE 5
Leah drives her car to the mechanic, then she takes the commuter rail train
back to her neighborhood. The average speed for the 10-mile trip is 15 miles
per hour faster on the train. Find an expression for Leah’s total travel time.
If she drove 30 mph, how long did this take?
Speed = r + 15
Speed = r
USE APPROPRIATE TOOLS
Use a table to organize
information and help create
an accurate model of
the situation.
Distance
Rate
Time
Car
10
r
Commuter Rail
10
r + 15
10
r
10
r + 15
Remember: distance = rate ∙ time,
so time = ______
​​  distance
​​.
rate
Total time for the trip:
10(r + 15) ________
10  ​​ = _________
___
______
​​  10
​ 
​ + ​​  10r ​​
r ​​ + ​​ 
r + 15
r(r + 15)
r(r + 15)
Add the times for each
part of the trip.
10r + 150 + 10r
______________
= ​​   
​​
r(r + 15)
= _________
​​  20r + 150 ​​
r(r + 15)
CONTINUED ON THE NEXT PAGE
LESSON 4-4 Adding and Subtracting Rational Expressions
219
Activity
Assess
EXAMPLE 5 CONTINUED
At a driving rate of 30 mph, you can find the total time.
20(30) + 150
30(30 + 15)
______
= ​​  750 ​​
1,350
5 ​​
= ​​ __
9
20r + 150 ___________
​​ _________
 ​​ =   
​​    ​​
r(r + 15)
Substitute 30 mph for
the rate, and simplify.
The expression for Leah’s total travel time is _______
​​ 20r + 150 ​​. The total time is ​​ __59 ​​ h, or
r(r + 15)
about 33 min.
Try It! 5. On the way to work Juan carpools with a fellow co-worker, then
takes the city bus back home in the evening. The average speed
of the 20-mile trip is 5 miles per hour faster in the carpool. Write
an expression that represents Juan’s total travel time.
EXAMPLE 6
Simplify a Compound Fraction
A compound fraction is in the form of a fraction and has one or
more fractions in the numerator and/or the denominator. How
can you write a simpler form of a compound fraction?
__
​  x1  ​  + _____
​  2  ​
x+ 1
 ​​
​​ ________
1
__
​   ​
y
Method 1 F ind the Least Common Multiple (LCM) of the fractions in the
numerator and denominator. Multiply the numerator and the
denominator by the LCM.
2  ​ ​ ∙ [x(x + 1)y]  
1 ​ + ​ _____
2
  ​[__
​  1x ​ + ​ _____
  ​ __
x x + 1 ​  
x + 1]
________
____________________
​​ 
 ​​
=
​​ 
   
  
 ​​
__
__
​  1y ​
​  1y ​ ∙ [x(x + 1)y]
Multiply the numerator and
the denominator by the LCD.
(x + 1)y + 2xy
x(x + 1)
​= _____________
  
​ 
 ​
se the Distributive Property
U
to eliminate the fractions.
xy + y + 2xy
x(x + 1)
​
=
​ ___________
  
 ​
Simplify.
(3x + 1)y
x(x + 1)
= ​ _________ ​​
Factor.
Method 2 E
xpress the numerator and denominator as single fractions. Then
multiply the numerator by the reciprocal of the denominator.
1 ​ + ​ _____
2
  ​ __
x x + 1 ​  
________
​​ 
 ​​
=
__
​  1y ​
x + 1 _____
1 ​ ∙ ​ _____
2
x
__
  ​ __
x x + 1 ​ + ​  x + 1 ​ ∙ ​  x ​
_______________
  
​​ 
 ​​
__
​  1y ​
(x + 1) + 2x
__________
​
x(x + 1)
__________
​ ​ 
=
__
​  1y ​
USE STRUCTURE
Simplify.
y
1
3x + 1 __
​
=
​ _______
​ ∙ ​ 
Remember that the fraction bar
separating the numerator and
denominator represents division.
x(x + 1)
(3x + 1)y
x(x + 1)
Multiply the numerator by the
reciprocal of the denominator.
________
​
=
​ 
 ​​
Using either method,
x ≠ −1, 0 and y ≠ 0.
  ​ __1x ​ + ____
​ x +2 1 ​  
_______
​​ 
 ​​
__
​  1y ​
Simplify.
1
____
​  x − 1 ​  
a. _________
​​    
 ​​
x+1
4
____
____
  ​  3 ​ + ​  x − 1 ​  
(3x + 1)y
x(x + 1)
is equal to _______
​​ 
​​ when
Try It! 6. Simplify each compound fraction.
220 TOPIC 4 Rational Functions
Multiply the numerator by
the LCD.
1
__
  2 − ​  x ​  
b. ​​ _____
 ​​
x + __
​  2x ​
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Concept
Summary
Assess
CONCEPT SUMMARY Find Sums and Differences of Rational Expressions
WORDS
To add or subtract rational
expressions with common
denominators, add the
numerators and keep the
denominator the same.
To add or subtract rational expressions
with different denominators, rewrite
each expression so that its denominator
is the LCD, then add or subtract the
numerators.
NUMBERS
3 __
1 ​ + __
​​ __
​  3 ​ + _____
​  1 + ​
= ​  4 ​​
__
​​  1 ​​ + __
​​  1  ​​ = ____
​​  1 ​​ + ____
​​  1  ​​
5
5
5
5
6
15
2∙3
3∙5
1∙5+1∙2
= ​​ __________
 ​​
2∙3∙5
ALGEBRA
_____
​​  x ​ + _____
​  5 ​ = _____
​  x + 5 ​
x+4
x+4
2
______
​​  x + 3 ​ + __________
​ 
x+4
​ ​​  2​ − 1
x
Rewrite the rational
expressions using
the LCD.
​ ​​  2​ − 3x + 2
x
x+3
2
​
=
​ ___________
  ​+ ___________
​    
(x + 1)(x − 1)
(x − 1)(x − 2)
(x + 3)(x − 2)
(x + 1)(x − 1)(x − 2)
2(x + 1)
(x + 1)(x − 1)(x − 2)
_________________
=​    
​
  ​+ _________________
​   
  ​
Do You UNDERSTAND?
1.
Do You KNOW HOW?
6. Find the sum of ____
​​ x +3 1 ​ + ____
​  x 11
​​.
+1
How do you
rewrite rational expressions to find sums
and differences?
ESSENTIAL QUESTION
Find the LCM of the polynomials.
2. Vocabulary In your own words, define
compound fraction and provide an example
of one.
3. Error Analysis A student added the rational
expressions as follows:
7(7)
x+7
5x ​ + _____
5x ​ + _____
​ _____
​  7 ​ = ​ _____
​ 
​ = _______
​  5x + 49 ​
x+7
x
x+7
7.​​x​​  2​ − ​y​​  2​and ​x​​  2​ − 2xy + ​y​​  2​
8.​5​x​​  3​y​and ​15​x​​  2​​y​​  2​​
Find the sum or difference.
y
4​y​​  ​ 10x
9y + 2
__________
3x ​ − ____
9.​​ ____
​ 
​
2
10.​​ 
x+7
Describe and correct the error the student
made.
4. Construct Arguments Explain why, when
stating the domain of a sum or difference
of rational expressions, not only should the
simplified sum or difference be considered
but the original expression should also be
considered.
3​y​​  2​ − 2y − 8
​ + __________
​  2 7
​
​3y​​  ​ + y − 4
11. Find the perimeter of the quadrilateral in
simplest form.
x
2
3x
1
x
2x
5. Make Sense and Persevere In adding or
subtracting rational expressions, why is the
L in LCD significant?
LESSON 4-4 Adding and Subtracting Rational Expressions
221
UNDERSTAND
13. Error Analysis Describe and correct the
error a student made in adding the rational
expressions.
1
+
2x
1
2x
=
+
3x + 3
(x + 2)(x + 1)
3(x + 1)
3
x(x + 2)
=
+
3(x + 2)(x + 1) 3(x + 2)(x + 1)
=
3 + x2 + 2x
3(x + 2)(x + 1)
=
x2 + 2x + 3
3(x + 2)(x + 1)
=
(x + 3) (x + 1)
•
3(x + 2) (x + 1)
=
x+3
3(x + 2)
✗
14. Higher Order Thinking Find the slope of the
line that passes through the points shown.
Express in simplest form.
4
2
−4 −2 O
−2
− 1 , −1
b
a
(
Find the sum.
Additional Exercises Available Online
y
x+7
x+7
3y − 1
9y + 6
_______ _______
18. ​​  y​ ​​  2​ + 4y ​​ + ​​  y(y + 4)​​
Find the LCM for each group of expressions.
SEE EXAMPLE 2
19. ​​x​​  2​ − 7x + 6, ​x​​  2​ − 5x − 6​
20. ​​y​​  2​ + 2y − 24​, ​​y​​  2​ − 16​, 2y
Find the sum.
SEE EXAMPLE 3
21. _______
​​  6x ​+ _______
​  4 ​​
​x​​  2​
− 8x
2x − 16
Find the difference.
3y
22.​​ _______
​+ ___
​  2 ​
2
​2y​​  ​ − y
SEE EXAMPLE 4
y−1
3y + 15
2y
y+3
5y + 25
24.​​ _______​ − _______
​ 
​​
25 On Saturday morning, Ahmed decided to take
a bike ride from one end of the 15-mile bike
trail to the other end of the bike trail and back.
His average speed the first half of the ride was
2 mph faster than his speed on the second half.
Find an expression for Ahmed’s total travel
time. If his average speed for the first half of
the ride was 12 mph, how long was Ahmed’s
bike ride? SEE EXAMPLE 5
Speed
x+2
( 1a , 1b ( x
2
SEE EXAMPLE 1
17. _____
​​  4x ​ + _____
​  9 ​​
23. ______
​​  24x ​​ − _____
​​  4 ​​
​x​​  ​ − 1 x − 1
(x + 3)(x + 1)
=
3(x + 2)(x + 1)
(
Tutorial
PRACTICE
12. Generalize Explain how addition and
subtraction of rational expressions is similar
to and different from addition and subtraction
of rational numbers.
x2 + 3x + 2
Practice
Scan for
Multimedia
PRACTICE & PROBLEM SOLVING
Trail length
15 miles
Speed x
4
−2
15. Reason For what values of x is the sum of
x − 5y
x + 7y
_____
​​  x + y ​​ and _____
​​  x + y ​​ undefined? Explain.
16. Error Analysis A student says that the LCM of ​​
3x​​  2​ + 7x + 2​and ​9x + 3​ is ​(​3x​​  2​ + 7x + 2)​
​(9x + 3)​. Describe and correct the error the
student made.
222 TOPIC 4 Rational Functions
Rewrite as a rational expression.
SEE EXAMPLE 6
  1 + __
​  1x ​  
26. _____
​​ 
 ​​
x − __
​  1x ​
3 ​ + ​ __
7
  ​ __
y x ​  
 ​​
27.​​ _____
2 ​
__
​  1 ​ − ​ __
1 ​ + __
1
   ​ __
a ​  b ​  
_______
 ​​
28. ​​  2
​​  2​
_______
​  ​a​​  ​− ​b
 ​
​z​​  ​ − z −12 ​  
__________
  ​ 
2​ − 2z −15
z
​
​​ 
___________
  
29.​​   
 ​​
​__________
z​​  2​ + 8z +12
​  2
 ​
ab
y
x
2
​z​​  ​ −5z −14
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Practice
PRACTICE & PROBLEM SOLVING
Mixed Review Available Online
ASSESSMENT PRACTICE
APPLY
30. Use Structure Aisha paddles a kayak 5 miles
downstream at a rate 3 mph faster than the
river’s current. She then travels 4 miles back
upstream at a rate 1 mph slower than the
river’s current. Hint: Let x represent the rate
of the river current.
33. Which of the following compound fractions
simplifies to ____
​​  xx +– 31 ​​? Select all that apply.
2
2
​x​​  ​ + 5x + 4
 ​   
   ​ __________
​ 
x​​  2​ + 2x −8  ​​
__________
Ⓐ​​   
  
2
_________
​  ​x​​  2​ − 4x + 3 ​
​x​​  ​ − 1 ​   
    ​ ______
2
​x​​  ​ − 4  ​​
Ⓑ​​ __________
2
__________
​  ​x​​  ​ + x − 7  ​
​ ​​  2​ + 5x + 6
x
​x​​  ​ − 3x + 2
2
2
​x​​  ​ + 3x − 10
  
 ​  
  ​ ___________
​x​​  2​ − 5x + 6 ​​
___________
Ⓓ​​   
  
2
_________
​  2​x​​  ​ − 25  ​
​x​​  ​ + 3x − 10
  
 ​  
  ​ ___________
​x​​  2​ − 16  ​​
___________
Ⓒ​​   
  
2
5
__________
​  ​x​​  ​ −2 4x − ​
Speed = x + 3
​x​​  ​ − 1
water current = x mph
a. Write and simplify an expression to represent
the total time it takes Aisha to paddle the
kayak 5 miles downstream and 4 miles
upstream.
b. If the rate of the river current, x, is 2 mph,
how long was Aisha’s entire kayak trip?
31. Model With Mathematics Rectangles A and B
are similar. An expression that represents the
width of each rectangle is shown. Find the scale
factor of rectangle A to rectangle B in simplest
form.
x2 − 25
x−4
Tutorial
A
B
x+5
x2 − 16
32. Reason The Taylor family drives 180 miles
(round trip) to a professional basketball game.
On the way to the game, their average speed is
approximately 8 mph faster than their speed on
the return trip home.
a. Let x represent their average speed on the
way home. Write and simplify an expression
to represent the total time it took them to
drive to and from the game.
b. If their average speed going to the game
was 72 mph, how long did it take them to
drive to the game and back?
​x​​  ​− 4x − 5
34. SAT/ACT What is the difference between
x−y
​​  6 ​​?
​​ __9x ​​ and ____
5x − y
Ⓐ​​ ______
​​
18
5x + y
Ⓑ​​ ______
​​
18
−x + 3y
Ⓒ​​ _______
​​
18
−x − 3y
Ⓓ​​ _______
​​
18
1
35. Performance Task The lens equation ​​ __1 ​​ = ​​ __
​​ + ___
​​  1 ​​
f
​d​  i​​
​d​  o​​
represents the relationship between f, the focal
length of a camera lens, ​​d​  i​​​, the distance from
the lens to the film, and ​​d​  o​​​, the distance from
the lens to the object.
Lens
Object
f
do
Film
f
di
Part A Find the focal length of a camera lens
if an object that is 12 cm from a camera lens is
in focus on the film when the lens is 6 cm from
the film.
Part B Suppose the focal length of another
camera lens is 3 inches, and the object to be
photographed is 5 feet away. What distance (to
the nearest tenth inch) should the lens be from
the film?
LESSON 4-4 Adding and Subtracting Rational Expressions
223
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