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Simplify: x 3 + 8 x 2 4 .

x 2 2 x + 4 x 2

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Simplify rational expressions with opposite factors

Now we will see how to simplify a rational expression whose numerator and denominator have opposite factors. Let’s start with a numerical fraction, say 7 −7 . We know this fraction simplifies to −1 . We also recognize that the numerator and denominator are opposites.

In Foundations , we introduced opposite notation: the opposite of a is a . We remember, too, that a = −1 · a .

We simplify the fraction a a , whose numerator and denominator are opposites, in this way:

a a We could rewrite this. 1 · a −1 · a Remove the common factors. 1 −1 Simplify. −1


So, in the same way, we can simplify the fraction x 3 ( x 3 ) :

We could rewrite this. 1 · ( x 3 ) −1 · ( x 3 ) Remove the common factors. 1 −1 Simplify. −1


But the opposite of x 3 could be written differently:

( x 3 ) Distribute. x + 3 Rewrite. 3 x

This means the fraction x 3 3 x simplifies to −1 .

In general, we could write the opposite of a b as b a . So the rational expression a b b a simplifies to −1 .

Opposites in a rational expression

The opposite of a b is b a .

a b b a = −1 a b

An expression and its opposite divide to −1 .

We will use this property to simplify rational expressions that contain opposites in their numerators and denominators.

Simplify: x 8 8 x .

Solution

x 8 8 x Recognize that x 8 and 8 x are opposites. −1

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Simplify: y 2 2 y .

−1

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Simplify: n 9 9 n .

−1

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Remember, the first step in simplifying a rational expression is to factor the numerator and denominator completely.

Simplify: 14 2 x x 2 49 .

Solution

.
Factor the numerator and denominator. .
Recognize that 7 x and x 7 are opposites . .
Simplify. .

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Simplify: 10 2 y y 2 25 .

2 y + 5 .

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Simplify: 3 y 27 81 y 2 .

3 9 + y

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Simplify: x 2 4 x 32 64 x 2 .

Solution

.
Factor the numerator and denominator. .
Recognize the factors that are opposites. .
Simplify. .

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Simplify: x 2 4 x 5 25 x 2 .

x + 1 x + 5

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Simplify: x 2 + x 2 1 x 2 .

x + 2 x + 1

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Key concepts

  • Determine the Values for Which a Rational Expression is Undefined
    1. Set the denominator equal to zero.
    2. Solve the equation, if possible.
  • Simplified Rational Expression
    • A rational expression is considered simplified if there are no common factors in its numerator and denominator.
  • Simplify a Rational Expression
    1. Factor the numerator and denominator completely.
    2. Simplify by dividing out common factors.
  • Opposites in a Rational Expression
    • The opposite of a b is b a .
      a b b a = −1 a 0 , b 0 , a b

Practice makes perfect

In the following exercises, determine the values for which the rational expression is undefined.


2 x z
4 p 1 6 p 5
n 3 n 2 + 2 n 8

z = 0 p = 5 6
n = −4 , n = 2

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10 m 11 n
6 y + 13 4 y 9
b 8 b 2 36

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4 x 2 y 3 y
3 x 2 2 x + 1
u 1 u 2 3 u 28

y = 0 x = 1 2
u = −4 , u = 7

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5 p q 2 9 q
7 a 4 3 a + 5
1 x 2 4

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Evaluate Rational Expressions

In the following exercises, evaluate the rational expression for the given values.

2 x x 1

x = 0
x = 2
x = −1

0 4 1

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4 y 1 5 y 3

y = 0
y = 2
y = −1

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2 p + 3 p 2 + 1

p = 0
p = 1
p = −2

3 5 2 1 5

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x + 3 2 3 x

x = 0
x = 1
x = −2

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y 2 + 5 y + 6 y 2 1

y = 0
y = 2
y = −2

−6 20 3 0

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z 2 + 3 z 10 z 2 1

z = 0
z = 2
z = −2

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a 2 4 a 2 + 5 a + 4

a = 0
a = 1
a = −2

−1 3 10 0

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b 2 + 2 b 2 3 b 4

b = 0
b = 2
b = −2

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x 2 + 3 x y + 2 y 2 2 x 3 y

  1. x = 1 , y = −1
  2. x = 2 , y = 1
  3. x = −1 , y = −2

0 3 4 15 4

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c 2 + c d 2 d 2 c d 3

  1. c = 2 , d = −1
  2. c = 1 , d = −1
  3. c = −1 , d = 2
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m 2 4 n 2 5 m n 3

  1. m = 2 , n = 1
  2. m = −1 , n = −1
  3. m = 3 , n = 2

0 3 5 7 20

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2 s 2 t s 2 9 t 2

  1. s = 4 , t = 1
  2. s = −1 , t = −1
  3. s = 0 , t = 2
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Simplify Rational Expressions

In the following exercises, simplify.

8 m 3 n 12 m n 2

2 m 2 3 n

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3 c 9 5 c 15

3 5

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12 p 240 5 p 100

12 5

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a 2 a 12 a 2 8 a + 16

a + 3 a 4

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x 2 + 4 x 5 x 2 2 x + 1

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y 2 + 3 y 4 y 2 6 y + 5

y + 4 y 5

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v 2 + 8 v + 15 v 2 v 12

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x 2 25 x 2 + 2 x 15

x 5 x 3

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a 2 4 a 2 + 6 a 16

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y 2 2 y 3 y 2 9

y + 1 y + 3

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b 2 + 9 b + 18 b 2 36

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y 3 + y 2 + y + 1 y 2 + 2 y + 1

y 2 + 1 y + 1

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p 3 + 3 p 2 + 4 p + 12 p 2 + p 6

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x 3 2 x 2 25 x + 50 x 2 25

x 2

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q 3 + 3 q 2 4 q 12 q 2 4

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3 a 2 + 15 a 6 a 2 + 6 a 36

a ( a + 5 ) 2 ( a + 3 ) ( a 2 )

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8 b 2 32 b 2 b 2 6 b 80

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−5 c 2 10 c −10 c 2 + 30 c + 100

c 2 ( c 5 )

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4 d 2 24 d 2 d 2 4 d 48

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3 m 2 + 30 m + 75 4 m 2 100

3 ( m + 5 ) 4 ( m 5 )

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5 n 2 + 30 n + 45 2 n 2 18

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5 r 2 + 30 r 35 r 2 49

5 ( r 1 ) r + 7

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3 s 2 + 30 s + 24 3 s 2 48

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t 3 27 t 2 9

t 2 + 3 t + 9 t + 3

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w 3 + 216 w 2 36

w 2 6 w + 36 w 6

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Simplify Rational Expressions with Opposite Factors

In the following exercises, simplify each rational expression.

12 2 x x 2 36

2 x + 6

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4 v 32 64 v 2

4 8 + v

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y 2 11 y + 24 9 y 2

y 8 3 + y

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z 2 9 z + 20 16 z 2

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a 2 5 z 36 81 a 2

a + 4 9 + a

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b 2 + b 42 36 b 2

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Everyday math

Tax Rates For the tax year 2015, the amount of tax owed by a single person earning between $37,450 and $90,750, can be found by evaluating the formula 0.25 x 4206.25 , where x is income. The average tax rate for this income can be found by evaluating the formula 0.25 x 4206.25 x . What would be the average tax rate for a single person earning $50,000?

16.5%

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Work The length of time it takes for two people for perform the same task if they work together can be found by evaluating the formula x y x + y . If Tom can paint the den in x = 45 minutes and his brother Bobby can paint it in y = 60 minutes, how many minutes will it take them if they work together?

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Writing exercises

Explain how you find the values of x for which the rational expression x 2 x 20 x 2 4 is undefined.

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Explain all the steps you take to simplify the rational expression p 2 + 4 p 21 9 p 2 .

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Self check

After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.

This figure shows a table with four columns and five rows. The first row is a header row and each column is labeled. The first column header is labeled “I can…”, the second is labeled “Confidently”, the third is labeled “With some help”, and the fourth is labeled “No—I don’t get it!” In the first column under “I can”, the cells read “determine the values for which a rational expression is undefined,” “evaluate rational expressions,” “simplify rational expressions,” and “simplify rational expressions with opposite factors.” The rest of the cells are blank.

If most of your checks were:

…confidently. Congratulations! You have achieved your goals in this section! Reflect on the study skills you used so that you can continue to use them. What did you do to become confident of your ability to do these things? Be specific!

…with some help. This must be addressed quickly as topics you do not master become potholes in your road to success. Math is sequential - every topic builds upon previous work. It is important to make sure you have a strong foundation before you move on. Who can you ask for help? Your fellow classmates and instructor are good resources. Is there a place on campus where math tutors are available? Can your study skills be improved?

…no - I don’t get it! This is critical and you must not ignore it. You need to get help immediately or you will quickly be overwhelmed. See your instructor as soon as possible to discuss your situation. Together you can come up with a plan to get you the help you need.

Questions & Answers

how did you get 1640
Noor Reply
If auger is pair are the roots of equation x2+5x-3=0
Peter Reply
Wayne and Dennis like to ride the bike path from Riverside Park to the beach. Dennis’s speed is seven miles per hour faster than Wayne’s speed, so it takes Wayne 2 hours to ride to the beach while it takes Dennis 1.5 hours for the ride. Find the speed of both bikers.
MATTHEW Reply
420
Sharon
from theory: distance [miles] = speed [mph] × time [hours] info #1 speed_Dennis × 1.5 = speed_Wayne × 2 => speed_Wayne = 0.75 × speed_Dennis (i) info #2 speed_Dennis = speed_Wayne + 7 [mph] (ii) use (i) in (ii) => [...] speed_Dennis = 28 mph speed_Wayne = 21 mph
George
Let W be Wayne's speed in miles per hour and D be Dennis's speed in miles per hour. We know that W + 7 = D and W * 2 = D * 1.5. Substituting the first equation into the second: W * 2 = (W + 7) * 1.5 W * 2 = W * 1.5 + 7 * 1.5 0.5 * W = 7 * 1.5 W = 7 * 3 or 21 W is 21 D = W + 7 D = 21 + 7 D = 28
Salma
Devon is 32 32​​ years older than his son, Milan. The sum of both their ages is 54 54​. Using the variables d d​ and m m​ to represent the ages of Devon and Milan, respectively, write a system of equations to describe this situation. Enter the equations below, separated by a comma.
Aaron Reply
find product (-6m+6) ( 3m²+4m-3)
SIMRAN Reply
-42m²+60m-18
Salma
what is the solution
bill
how did you arrive at this answer?
bill
-24m+3+3mÁ^2
Susan
i really want to learn
Amira
I only got 42 the rest i don't know how to solve it. Please i need help from anyone to help me improve my solving mathematics please
Amira
Hw did u arrive to this answer.
Aphelele
hi
Bajemah
-6m(3mA²+4m-3)+6(3mA²+4m-3) =-18m²A²-24m²+18m+18mA²+24m-18 Rearrange like items -18m²A²-24m²+42m+18A²-18
Salma
complete the table of valuesfor each given equatio then graph. 1.x+2y=3
Jovelyn Reply
x=3-2y
Salma
y=x+3/2
Salma
Hi
Enock
given that (7x-5):(2+4x)=8:7find the value of x
Nandala
3x-12y=18
Kelvin
please why isn't that the 0is in ten thousand place
Grace Reply
please why is it that the 0is in the place of ten thousand
Grace
Send the example to me here and let me see
Stephen
A meditation garden is in the shape of a right triangle, with one leg 7 feet. The length of the hypotenuse is one more than the length of one of the other legs. Find the lengths of the hypotenuse and the other leg
Marry Reply
how far
Abubakar
cool u
Enock
state in which quadrant or on which axis each of the following angles given measure. in standard position would lie 89°
Abegail Reply
hello
BenJay
hi
Method
I am eliacin, I need your help in maths
Rood
how can I help
Sir
hmm can we speak here?
Amoon
however, may I ask you some questions about Algarba?
Amoon
hi
Enock
what the last part of the problem mean?
Roger
The Jones family took a 15 mile canoe ride down the Indian River in three hours. After lunch, the return trip back up the river took five hours. Find the rate, in mph, of the canoe in still water and the rate of the current.
cameron Reply
Shakir works at a computer store. His weekly pay will be either a fixed amount, $925, or $500 plus 12% of his total sales. How much should his total sales be for his variable pay option to exceed the fixed amount of $925.
mahnoor Reply
I'm guessing, but it's somewhere around $4335.00 I think
Lewis
12% of sales will need to exceed 925 - 500, or 425 to exceed fixed amount option. What amount of sales does that equal? 425 ÷ (12÷100) = 3541.67. So the answer is sales greater than 3541.67. Check: Sales = 3542 Commission 12%=425.04 Pay = 500 + 425.04 = 925.04. 925.04 > 925.00
Munster
difference between rational and irrational numbers
Arundhati Reply
When traveling to Great Britain, Bethany exchanged $602 US dollars into £515 British pounds. How many pounds did she receive for each US dollar?
Jakoiya Reply
how to reduced echelon form
Solomon Reply
Jazmine trained for 3 hours on Saturday. She ran 8 miles and then biked 24 miles. Her biking speed is 4 mph faster than her running speed. What is her running speed?
Zack Reply
d=r×t the equation would be 8/r+24/r+4=3 worked out
Sheirtina
Practice Key Terms 1

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Source:  OpenStax, Elementary algebra. OpenStax CNX. Jan 18, 2017 Download for free at http://cnx.org/content/col12116/1.2
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