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13.9.3: Add and Subtract Rational Expressions with a Common Denominator

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Learning Objectives

By the end of this section, you will be able to:

  • Add rational expressions with a common denominator
  • Subtract rational expressions with a common denominator
  • Add and subtract rational expressions whose denominators are opposites
Note

Before you get started, take this readiness quiz.

If you miss a problem, go back to the section listed and review the material.

  1. Add: y3+93.
    If you missed this problem, review Exercise 1.7.1.
  2. Subtract: 10x2x.
    If you missed this problem, review Exercise 1.7.7.
  3. Factor completely: 8n520n3.
    If you missed this problem, review Exercise 7.5.1.
  4. Factor completely: 45a35ab2.
    If you missed this problem, review Exercise 7.5.10.

Add Rational Expressions with a Common Denominator

What is the first step you take when you add numerical fractions? You check if they have a common denominator. If they do, you add the numerators and place the sum over the common denominator. If they do not have a common denominator, you find one before you add.

It is the same with rational expressions. To add rational expressions, they must have a common denominator. When the denominators are the same, you add the numerators and place the sum over the common denominator.

Definition: RATIONAL EXPRESSION ADDITION

If p, q, and r are polynomials where r0, then

pr+qr=p+qr

To add rational expressions with a common denominator, add the numerators and place the sum over the common denominator.

We will add two numerical fractions first, to remind us of how this is done.

Example 13.9.3.1

Add: 518+718.

Answer
  518+718
The fractions have a common denominator, so add the numerators and place the sum over the common denominator. 5+718
Add in the numerator. 1218
Factor the numerator and denominator to show the common factors. 6·26·3
Simplify. 23
Example 13.9.3.2

Add: 716+516.

Answer

34

Example 13.9.3.3

Add: 310+110.

Answer

25

Remember, we do not allow values that would make the denominator zero. What value of yy should be excluded in the next example?

Example 13.9.3.4

Add: 3y4y3+74y3.

Answer
  3y4y3+74y3.
The fractions have a common denominator, so add the numerators and place the sum over the common denominator. 3y+74y3
The numerator and denominator cannot be factored. The fraction is simplified.  
Example 13.9.3.5

Add: 5x2x+3+22x+3.

Answer

5x+22x+3.

Example 13.9.3.6

Add: xx2+1x2.

Answer

x+1x2

Example 13.9.3.7

Add: 7x+12x+3+x2x+3.

Answer
  7x+12x+3+x2x+3
The fractions have a common denominator, so add the numerators and place the sum over the common denominator. 7x+12+x2x+3
Write the degrees in descending order. x2+7x+12x+3
Factor the numerator. (x+3)(x+4)x+3
Simplify. x+4
Example 13.9.3.8

Add: 9x+14x+7+x2x+7.

Answer

x+2

Example 13.9.3.9

Add: x2+8xx+5+15x+5.

Answer

x+3

Subtract Rational Expressions with a Common Denominator

To subtract rational expressions, they must also have a common denominator. When the denominators are the same, you subtract the numerators and place the difference over the common denominator.

Definition: RATIONAL EXPRESSION SUBTRACTION

If p, q, and r are polynomials where r0

prqr=pqr

To subtract rational expressions, subtract the numerators and place the difference over the common denominator.

We always simplify rational expressions. Be sure to factor, if possible, after you subtract the numerators so you can identify any common factors.

Example 13.9.3.10

Subtract: n2n10100n10.

Answer
  n2n10100n10
The fractions have a common denominator, so add the numerators and place the sum over the common denominator. n2100n10
Factor the numerator. (n10)(n+10)n10
Simplify. n+10
Example 13.9.3.11

Subtract: x2x+39x+3.

Answer

x−3

Example 13.9.3.12

Subtract: 4x22x5252x5.

Answer

2x+5

Be careful of the signs when you subtract a binomial!

Example 13.9.3.13

Subtract: y2y62y+24y6.

Answer
  y2y62y+24y6
The fractions have a common denominator, so add the numerators and place the sum over the common denominator. y2(2y+24)y6
Distribute the sign in the numerator. y22y24y6
Factor the numerator. (y6)(y+4)y6
Simplify. y+4
Example 13.9.3.14

Subtract: n2n4n+12n4.

Answer

n+3

Example 13.9.3.15

Subtract: y2y19y8y1.

Answer

y−8

Example 13.9.3.16

Subtract: 5x27x+3x23x184x2+x9x23x18.

Answer
  5x27x+3x23x+184x2+x9x23x+18
The fractions have a common denominator, so add the numerators and place the sum over the common denominator. 5x27x+3(4x2+x9)x23x+18
Distribute the sign in the numerator. 5x27x+34x2x+9x23x+18
Combine like terms. x28x+12x23x+18
Factor the numerator and the denominator. (x2)(x6)(x+3)(x6)
Simplify. x2x+3
Example 13.9.3.17

Subtract: 4x211x+8x23x+23x2+x3x23x+2.

Answer

x11x2

Example 13.9.3.18

Subtract: 6x2x+20x2815x2+11x7x281.

Answer

x3x+9

Add and Subtract Rational Expressions whose Denominators are Opposites

When the denominators of two rational expressions are opposites, it is easy to get a common denominator. We just have to multiply one of the fractions by 11

Let’s see how this works.

  .
Multiply the second fraction by 11. .
The denominators are the same. .
Simplify. .
Example 13.9.3.19

Add: 4u13u1+u13u.

Answer
  .
Multiply the second fraction by 11. .
Simplify the second fraction. .
The denominators are the same. Add the numerators. .
Simplify. .
Simplify. .
Example 13.9.3.20

Add: 8x152x5+2x52x.

Answer

3

Example 13.9.3.21

Add: 6y2+7y104y7+2y2+2y+1174y.

Answer

y+3

Example 13.9.3.22

Subtract: m26mm213m+21m2.

Answer
  .
Multiply the second fraction by 11. .
Simplify the second fraction. .
The denominators are the same. Subtract the numerators. .
Distribute. m2−6m+3m+2m2−1
Combine like terms. .
Factor the numerator and denominator. .
Simplify by removing common factors. .
Simplify. .
Example 13.9.3.23

Subtract: y25yy246y64y2.

Answer

y+3y+2

Example 13.9.3.24

Subtract: 2n2+8n1n21n27n11n2.

Answer

3n2n1

Key Concepts

  • Rational Expression Addition
    • If p, q, and r are polynomials where r0, then

      pr+qr=p+qr

    • To add rational expressions with a common denominator, add the numerators and place the sum over the common denominator.
  • Rational Expression Subtraction
    • If p, q, and r are polynomials where r0

      prqr=pqr

    • To subtract rational expressions, subtract the numerators and place the difference over the common denominator.

This page titled 13.9.3: Add and Subtract Rational Expressions with a Common Denominator is shared under a CC BY license and was authored, remixed, and/or curated by OpenStax.

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