Adding and Subtracting Rational Expressions: Principles and Examples, Study notes of Algebra

An in-depth explanation of the principles and techniques for adding and subtracting rational expressions. It includes multiple examples with detailed solutions, as well as a section on avoiding common errors.

Typology: Study notes

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ADDING AND SUBTRACTING RATIONAL EXPRESSIONS
ADDING AND SUBTRACTING RATIONAL EXPRESSIONSADDING AND SUBTRACTING RATIONAL EXPRESSIONS
ADDING AND SUBTRACTING RATIONAL EXPRESSIONS
To
To To
To
Add or Subtract Two Fractions
Add or Subtract Two FractionsAdd or Subtract Two Fractions
Add or Subtract Two Fractions
Example 1
Example 1Example 1
Example 1 a)
a) a)
a) Add
&
'(
)
*
'(
b)
b) b)
b) Subtract
+
,
-
'
,
Solution: a)
a) a)
a)
&
'(
)
*
'(
/
& 0 *
'(
/
'+
'(
b)
b)b)
b)
+
,
-
'
,
/
+ 1 '
,
/
2
,
The same principles apply when adding
addingadding
adding or subtracting rational expressions
subtracting rational expressionssubtracting rational expressions
subtracting rational expressions containing
variables.
To
To To
To
Add or Subtract Rational Expressions with a Common Denominator
Add or Subtract Rational Expressions with a Common DenominatorAdd or Subtract Rational Expressions with a Common Denominator
Add or Subtract Rational Expressions with a Common Denominator
1.
Add or subtract the numerators
.
2.
the sum or difference of the numerators found in step 1 over
the common denominator
.
3.
Simplify the faction if possible
.
Example 2
Example 2Example 2
Example 2 Add
(
; 1 +
)
; 0 <
; 1 +
Solution:
(
; 1 +
)
; 0 <
; 1 +
/
( 0=;0<)
; 1 +
/
; 0 *
; 1 +
Example
Example Example
Example 3
33
3 Add
;
>
0?;1<
=;0+)=;1<)
)
2;0'<
=;0+)=;1<)
Solution:
;
>
0?;1<
=;0+)=;1<)
)
2;0'<
=;0+)=;1<)
/
@;
>
0?;1<A0 =2;0'<)
=;0+)=;1<)
Write as a single fraction.
Remove parentheses
/
;
>
0?;1< 0 2;0'<
=;0+)=;1<)
in the numerator.
/
;
>
0&;0'C
=;0+)=;1<)
Combine like terms.
/
=;0+)=;0<)
=;0+)=;1<)
Factor.
/
=;0+)=;0<)
=;0+)=;1<)
/
=;0<)
=;1<)
Divide out common factors.
When subtracting rational expressions, n
When subtracting rational expressions, nWhen subtracting rational expressions, n
When subtracting rational expressions, note that the
ote that theote that the
ote that the
entire numerator of the second fraction
entire numerator of the second fractionentire numerator of the second fraction
entire numerator of the second fraction
=not just the first term) must be subtracted.
must be subtracted.must be subtracted.
must be subtracted. Also note that the sign of
each
term of the
numerator being subtracted will change when the parentheses are removed.
Example
Example Example
Example 4
44
4 Subtract
;
>
1<;0?
;
>
0&;0'<
-
;
>
1 2;1+
;
>
0&;0'<
H
I
)
J
I
/
H
)
J
I
,
I
K
0
H
I
-
J
I
/
H
-
J
I
,
I
K
0
pf3
pf4
pf5
pf8

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ADDING AND SUBTRACTING RATIONAL EXPRESSIONSADDING AND SUBTRACTING RATIONAL EXPRESSIONSADDING AND SUBTRACTING RATIONAL EXPRESSIONSADDING AND SUBTRACTING RATIONAL EXPRESSIONS

ToToToTo Add or Subtract Two FractionsAdd or Subtract Two FractionsAdd or Subtract Two FractionsAdd or Subtract Two Fractions

Example 1Example 1Example 1Example 1 a)a)a)a) Add

⡩⡴ ㎗^

⡩⡴ b)b)b)b) Subtract^

⡷ ㎘^

Solution: a)a)a)a)

⡩⡴ ㎗^

⡩⡴ 㐄^

⡩⡴ 㐄^

⡩⡴ b)b)b)b)^

⡷ ㎘^

⡷ 㐄^

⡷ 㐄^

The same principles apply when addingaddingaddingadding or subtracting rational expressionssubtracting rational expressionssubtracting rational expressionssubtracting rational expressions containing

variables.

ToToToTo Add or Subtract Rational Expressions with a Common DenominatorAdd or Subtract Rational Expressions with a Common DenominatorAdd or Subtract Rational Expressions with a Common DenominatorAdd or Subtract Rational Expressions with a Common Denominator

1. Add or subtract the numerators.

2. Place the sum or difference of the numerators found in step 1 over

the common denominator.

3. Simplify the faction if possible.

Example 2Example 2Example 2Example 2 Add

け ⡹ ⡳ ㎗^

Solution:

け ⡹ ⡳ ㎗^

け ⡹ ⡳ 㐄^

け ⡹ ⡳ 㐄^

ExampleExampleExampleExample 3 333 Add

䙦け⡸⡳)䙦け⡹⡰) ㎗^

Solution:

䙦け⡸⡳)䙦け⡹⡰) ㎗^

䙦け⡸⡳)䙦け⡹⡰) 㐄^

䙦け⡸⡳)䙦け⡹⡰)^ Write as a single fraction.

Remove parentheses

䙦け⡸⡳)䙦け⡹⡰)^ in the numerator.

䙦け⡸⡳)䙦け⡹⡰)^ Combine like terms.

䙦け⡸⡳)䙦け⡹⡰)^ Factor.

䙦け⡸⡳)䙦け⡹⡰) 㐄^

䙦け⡹⡰)^ Divide out common factors.

When subtracting rational expressions, nWhen subtracting rational expressions, nWhen subtracting rational expressions, nWhen subtracting rational expressions, note that theote that theote that theote that the entire numerator of the second fractionentire numerator of the second fractionentire numerator of the second fractionentire numerator of the second fraction

䙦not just the first term) must be subtracted.must be subtracted.must be subtracted.must be subtracted. Also note that the sign of each term of the

numerator being subtracted will change when the parentheses are removed.

ExampleExampleExampleExample 4 444 Subtract

けㄘ⡸⡵け⡸⡩⡰ ㎘^

Solution:

けㄘ⡸⡵け⡸⡩⡰ ㎘^

けㄘ⡸⡵け⡸⡩⡰ 㐄^

けㄘ⡸⡵け⡸⡩⡰^ Write as a single fraction.

Remove parentheses

けㄘ⡸⡵け⡸⡩⡰^ in the numerator.

けㄘ⡸⡵け⡸⡩⡰^ Combine like terms.

䙦け⡸⡱)䙦け⡸⡲)^ Factor.

䙦け⡸⡱)䙦け⡸⡲) 㐄^

け⡸⡱^ Divide out common factors.

FINDING THE LEAST COMMON DENOMINATORFINDING THE LEAST COMMON DENOMINATORFINDING THE LEAST COMMON DENOMINATORFINDING THE LEAST COMMON DENOMINATOR

When you add or subtract fractions, you begin by finding a common denominator. This is

what you must do when you add or subtract rational expressions. We seek the least

common denominator, LCD, because this produces the easiest calculations.

You can add or subtract rational expressions only when you have a common denominator.

Here the least common denominatorleast common denominatorleast common denominatorleast common denominator, LCD, of a collection of denominators is the smallest

expression that is divisible by each of the given denominators. This is equivalent to saying

that the least common denominator is the least common multiple, LCM, of all denominators

in the collection.

To Find the Least Common Denominator of Rational ExpressionsTo Find the Least Common Denominator of Rational ExpressionsTo Find the Least Common Denominator of Rational ExpressionsTo Find the Least Common Denominator of Rational Expressions

1. Factor each denominator completely. Any factors that occur more

than once should be expressed as powers. For example,

䙦ᡶ ㎘ 3 )䙦ᡶ ㎘ 3 ) should be expressed as 䙦ᡶ ㎘ 3 )⡰

2. List all different factors 䙦other than 1) that appear in any of the

denominators. When the same factor appears in more than one

denominator, write that factor with the highest power that

appears.

3. The least common denominator is the product of all the factors

listed in step 2.

Example 1Example 1Example 1Example 1 Find the least common denominator.

⡳ ㎗^

Solution: The only factor 䙦other than 1) of the first denominator is 5.

The only factor 䙦other than 1) of the second denominator is y.

The LCD is therefore 5 · y 㐄 5y.

Example 2Example 2Example 2Example 2 Find the LCD.

けㄘ^ ㎘^

Solution: The factors that appear in the denominators are 7 and x. List each factor

with its highest power. The LCD is the product of these factors.

Highest power of x

LCD 㐄 7 · ᡶ⡰^ 㐄 7ᡶ⡰

We need additional factors of 3 and m㎗n in our denominator. Hence, we multiply

the numerator and denominator by them to obtain the desired denominator. Thus

⡰䙦぀⡹ぁ) x^

⡱䙦぀⡸ぁ) 㐄^

⡴䙦぀⡸ぁ)䙦぀⡹ぁ) or^

Example 3Example 3Example 3Example 3 Write

げ⡸⡰ as an equivalent fraction with a denominator of ㎘2ᡷ

Solution: ㎘2ᡷ⡰^ ㎘ 10ᡷ ㎘ 12 factors into -2䙦y㎗3)䙦y㎗2). We must multiply the

numerator and denominator by -2䙦y㎗3). Hence

げ⡸⡰ x^

⡹⡰䙦げ⡸⡱) 㐄^

The total process of adding or subtracting rational expressions uses finding the LCD and

writing equivalent fractions. The complete list of steps is below.

ToToToTo Add or Subtract Two Rational Expressions with Unlike DenominatorsAdd or Subtract Two Rational Expressions with Unlike DenominatorsAdd or Subtract Two Rational Expressions with Unlike DenominatorsAdd or Subtract Two Rational Expressions with Unlike Denominators

1. Determine the LCD.

2. Rewrite each fraction as an equivalent fraction with the LCD. This

is done by multiplying both the numerator and denominator of

each fraction by any factors needed to obtain the LCD.

3. Add or subtract the numerators white maintaining the LCD.

4. When possible, factor the remaining numerator and simplify the

fraction.

Example 1Example 1Example 1Example 1 Add

け ㎗^

Solution: First we determine the LCD. The LCD 㐄 xy.

We write each fraction with the LCD. We do this by multiplying bothbothbothboth the numerator

and denominator of eacheacheacheach fraction by any factors needed to obtain the LCD.

In this problem, the fraction on the left must be multiplied by

げ and the fraction on

the right must be multiplied by

け ㎗^

げ 㐄^

げ䙳 ㎗^

け䙳 㐄^

けげ ㎗^

By multiplying both the numerator and denominator by the same factor, we are in

effect multiplying by 1, which does not change the value of the fraction, only its

appearance. Thus, the new fraction is equivalent to the original fraction.

Now we add the numerators, while leaving the LCD alone.

けげ ㎗^

けげ 㐄^

けげ or^

Example 2Example 2Example 2Example 2 Add

⡲けㄘげ ㎗^

Solution: The LCD is 28ᡶ⡰ᡷ⡱. We must write each fraction with the denominator

28ᡶ⡰ᡷ⡱. To do this, we multiply the fraction on the left by

⡵げㄘ^ and the fraction on the

right by

⡲けㄘげ ㎗^

⡩⡲けげㄙ^ 㐄^

⡵げㄘ䙳 ㎗^

⡩⡲けげㄙ^ · 䙲

⡰け䙳 㐄^

⡰⡶けㄘげㄙ^ ㎗^

⡰⡶けㄘげㄙ^ 㐄^

⡰⡶けㄘげㄙ^ or^

In this example, we multiplied the first fraction by

⡵げㄘ^ and the second fraction by^

to get two fractions with a common denominator. How did we know what to

multiply each fraction by? You can determine this by observing the LCD and then

determining what each denominator needs to be multiplied by to get the LCD. If this

is not obvious, you can divide the LCD by the given denominator to determine what

the numerator and denominator of each fraction should be multiplied by. In

Example 2, the LCD is 28ᡶ⡰ᡷ⡱. If we divide 28ᡶ⡰ᡷ⡱^ by each given denominator,

4ᡶ⡰ᡷ and 14ᡶᡷ⡱, we can determine what the numerator and denominator of each

respective fraction should be multiplied by.

⡩⡲けげㄙ^ 㐄 2ᡶ

Thus,

⡲けㄘげ should be multiplied by^

⡵げㄘ^ and^

⡩⡲けげㄙ^ should be multiplied by^

⡰け to obtain

the LCD 28ᡶ⡰ᡷ⡱.

Example 3Example 3Example 3Example 3 Subtract

け⡸⡳ ㎘^

Solution: The LCD is 䙦x㎗5)䙦x-3). The fraction on the left must be multiplied by

to obtain the LCD. The fraction on the right must be multiplied by

け⡸⡳ to obtain the

LCD.

け⡸⡳ ㎘^

け⡹⡱ 㐄^

け⡹⡱䙳 ㎘^

䙦け⡹⡱)䙦け⡸⡳) –^

䙦け⡹⡱)䙦け⡸⡳)^ Rewrite each fraction as an equivalent fraction with the LCD.

䙦け⡹⡱)䙦け⡸⡳) ㎘^

䙦け⡹⡱)䙦け⡸⡳)^ Distributive property.

䙦け⡹⡱)䙦け⡸⡳)^ Write as a single fraction.

䙦け⡹⡱)䙦け⡸⡳)^ Remove parentheses in the numerator.

䙦け⡹⡱)䙦け⡸⡳)^ Combine like terms in the numerator.

When adding or subtracting fractions whose denominators are opposites 䙦and therefore

differ only in signs), multiply both the numerator and denominator of either of the

fractions by -1. Then both fractions will have the same denominator.

ADDINGADDINGADDINGADDING &&& SUBTRACTING&SUBTRACTINGSUBTRACTINGSUBTRACTING RATIONAL EXPRESSIONS PRACTICERATIONAL EXPRESSIONS PRACTICERATIONAL EXPRESSIONS PRACTICERATIONAL EXPRESSIONS PRACTICE

⡱ ㎗^

⡵ ㎘^

け ㎗^

⡲け ㎗^

⡰⡲け ㎗^

け⡸⡳ ㎘^

け⡸⡲ ㎘^

⡱䙦け⡸⡶) ㎗^

⡰䙦け⡹⡷) ㎗^

⡱䙦け⡹⡩) ㎗^

け⡸⡳ ㎘^

けㄘ⡹⡲ ㎘^

けㄘ⡹⡴⡲ ㎘^

けㄘ⡸⡲け⡹⡩⡰ ㎘^

けㄘ⡹⡳け⡸⡲ ㎗^

けㄘ⡹⡴け⡹⡵ ㎗^

けㄘ⡹⡷ ㎘^

けㄘ⡹⡰⡳ ㎘^

⡱け ㎗^

け ㎗^

け ㎗^

け ㎗^

⡳けㄙげ ㎘^

けげ ㎗^

⡲け ㎘^

⡱け ㎘^

け⡹⡰ ㎘^

け⡹⡴ ㎘^

け⡹⡵ ㎘^

けㄘ⡸⡳け⡸⡴ ㎗^

けㄘ⡸⡲け⡸⡱ ㎘^

けㄘ⡸⡱け⡸⡰ ㎗^

けㄘ⡹⡰け⡹⡶ ㎘^

けㄘ⡸⡱け⡹⡲ ㎗^

けㄘ⡸⡴け⡸⡷ ㎗^

けㄘ⡸⡰け⡹⡱ ㎘^

けㄘ⡸⡰け⡹⡰⡲ ㎘^

けㄘ⡹⡳け⡸⡲ ㎗^

けㄘ⡹⡵け⡸⡴ ㎗^

け⡸⡰ ㎘^

け⡹⡱ ㎘^

け⡸⡩ ㎘^

け⡹⡰ ㎗^

け⡸⡱ ㎘^

け⡹⡩ ㎗^

けㄘ⡹⡩ ㎗^

⡩⡹け ㎘^

⡩⡹⡰け ㎘^

⡰け⡸⡩ ㎗^

けㄘ⡸⡳け⡸⡴ ㎘^

けㄘ⡸⡱け⡸⡰ ㎗^

けㄘ⡹⡳け⡸⡴ ㎘^

けㄘ⡹⡰け⡹⡱ ㎗^

PRACTICE ANSWERSPRACTICE ANSWERSPRACTICE ANSWERSPRACTICE ANSWERS