Adding and Subtracting Rational Expressions - Cobb … · Adding and Subtracting Rational...

Holt McDougal Algebra 2

Adding and Subtracting

Rational Expressions

Holt Algebra 2

Warm Up

Lesson Presentation

Lesson Quiz

Warm UpAdd or subtract.

x ≠ 0

x ≠ –1, x ≠ 1

Simplify. Identify any x-values for which the expression is undefined.

x – 1x2 – 1

1x + 1

Add and subtract rational expressions.

Simplify complex fractions.

Objectives

complex fraction

Vocabulary

Adding and subtracting rational expressions is similar to adding and subtracting fractions. To add or subtract rational expressions with like denominators, add or subtract the numerators and use the same denominator.

Add or subtract. Identify any x-values for which the expression is undefined.

Example 1A: Adding and Subtracting Rational

Expressions with Like Denominators

Add the numerators.

The expression is undefined at x = –4 because this value makes x + 4 equal 0.

x – 3x + 4

+x – 2x + 4

x – 3 +x + 4

x – 2

2x – 5x + 4

Combine like terms.

Example 1B: Adding and Subtracting Rational

Expressions with Like Denominators

Subtract the numerators.

There is no real value of x for which x2 + 1 = 0; the expression is always defined.

3x – 4x2 + 1

–6x + 1x2 + 1

3x – 4 –x2 + 1

(6x + 1)

–3x – 5x2 + 1 Combine like terms.

Distribute the negative sign.3x – 4 –

x2 + 16x – 1

Check It Out! Example 1a

Add the numerators.

6x + 5x2 – 3

+3x – 1x2 – 3

6x + 5 +x2 – 3

3x – 1

9x + 4x2 – 3

Combine like terms.

The expression is undefined at x = ± because this value makes x2 – 3 equal 0.

Check It Out! Example 1b

3x2 – 53x – 1

–2x2 – 3x – 2

3x – 1

3x – 13x2 – 5 – (2x2 – 3x – 2)

x2 + 3x – 33x – 1 Combine like terms.

Distribute the negative sign.3x2 – 5 –

3x – 12x2 + 3x + 2

The expression is undefined at x = because this value makes 3x – 1 equal 0.

To add or subtract rational expressions with unlike denominators, first find the least common denominator (LCD). The LCD is the least common multiple of the polynomials in the denominators.

Find the least common multiple for each pair.

Example 2: Finding the Least Common Multiple of

Polynomials

A. 4x2y3 and 6x4y5

4x2y3 = 2 2 x2 y3

6x4y5 = 3 2 x4 y5

The LCM is 2 2 3 x4 y5, or 12x4y5.

B. x2 – 2x – 3 and x2 – x – 6

x2 – 2x – 3 = (x – 3)(x + 1)

x2 – x – 6 = (x – 3)(x + 2)

The LCM is (x – 3)(x + 1)(x + 2).

Check It Out! Example 2

Find the least common multiple for each pair.

a. 4x3y7 and 3x5y4

4x3y7 = 2 2 x3 y7

3x5y4 = 3 x5 y4

The LCM is 2 2 3 x5 y7, or 12x5y7.

b. x2 – 4 and x2 + 5x + 6

x2 – 4 = (x – 2)(x + 2)

x2 + 5x + 6 = (x + 2)(x + 3)

The LCM is (x – 2)(x + 2)(x + 3).

To add rational expressions with unlike denominators, rewrite both expressions with the LCD. This process is similar to adding fractions.

Add. Identify any x-values for which the expression is undefined.

Example 3A: Adding Rational Expressions

x – 3x2 + 3x – 4

x – 3(x + 4)(x – 1)

x + 4Factor the denominators.

The LCD is (x + 4)(x – 1),

so multiply by . 2x x + 4

x – 1x – 1

x – 3(x + 4)(x – 1)

x + 4x – 1x – 1

x – 3 + 2x(x – 1)(x + 4)(x – 1)

Add the numerators.

2x2 – x – 3(x + 4)(x – 1)

Simplify the numerator.

Example 3A Continued

Write the sum in

factored or expanded

2x2 – x – 3(x + 4)(x – 1)

2x2 – x – 3x2 + 3x – 4

The expression is undefined at x = –4 and x = 1 because these values of x make the factors (x + 4) and (x – 1) equal 0.

x x + 2

x2 – 4

x x + 2

(x + 2)(x – 2)Factor the denominator.

The LCD is (x + 2)(x – 2),

so multiply by . x x + 2

x – 2x – 2

Example 3B: Adding Rational Expressions

x – 2x – 2

x x + 2

(x + 2)(x – 2)

x(x – 2) + (–8) (x + 2)(x – 2)

Add the numerators.

x2 – 2x – 8 (x + 2)(x – 2)

Write the numerator in

standard form.

Example 3B Continued

(x + 2)(x – 4)(x + 2)(x – 2)

Factor the numerator.

x – 4x – 2

Divide out common

factors.

The expression is undefined at x = –2 and x = 2 because these values of x make the factors (x + 2) and (x – 2) equal 0.

3x 2x – 2

+3x – 23x – 3

Factor the denominators.

The LCD is 6(x – 1), so

multiply by 3 and

3x 2(x – 1)

3x – 23(x – 1)

3x 2(x – 1)

+3x – 2

3(x – 1)

3x 2(x – 1)

+3x – 2

3(x – 1)22

Check It Out! Example 3a Continued

15x – 46(x – 1)

Simplify the numerator.

The expression is undefined at x = 1 because this value of x make the factor (x – 1) equal 0.

Add the numerators.9x + 6x – 4

6(x – 1)

2x + 6x2 + 6x + 9

2x + 6(x + 3)(x + 3)

The LCD is (x + 3)(x + 3),

so multiply by .x

(x + 3)(x + 3) (x + 3)

2x + 6(x + 3)(x + 3)

x + 3x + 3x + 3

Add the numerators.x2 + 3x + 2x + 6

(x + 3)(x + 3)

Check It Out! Example 3b Continued

standard form.

(x + 3)(x + 2)(x + 3)(x + 3)

x + 2x + 3

Divide out common

factors.

The expression is undefined at x = –3 because this value of x make the factors (x + 3) and (x + 3) equal 0.

x2 + 5x + 6 (x + 3)(x + 3)

Example 4: Subtracting Rational Expressions

Subtract . Identify any x-

values for which the expression is undefined.

2x2 – 30

x2 – 9–

2x2 – 30

(x – 3)(x + 3)–

The LCD is (x – 3)(x + 3),

so multiply by .x + 5 x + 3

(x – 3) (x – 3)

2x2 – 30

(x – 3)(x + 3)–

x + 5 x – 3x – 3

2x2 – 30 – (x + 5)(x – 3)(x – 3)(x + 3)

2x2 – 30 – (x2 + 2x – 15)(x – 3)(x + 3)

Multiply the binomials in

the numerator.

Example 4 Continued

2x2 – 30

x2 – 9–

2x2 – 30 – x2 – 2x + 15(x – 3)(x + 3)

Distribute the negative sign.

x2 – 2x – 15(x – 3)(x + 3)

standard form.

(x + 3)(x – 5)(x – 3)(x + 3) Factor the numerator.

x – 5x – 3 Divide out common factors.

The expression is undefined at x = 3 and x = –3 because these values of x make the factors (x + 3) and (x – 3) equal 0.

3x – 2

2x + 5–

5x – 2

(3x – 2)(5x – 2) – 2(2x + 5)(2x + 5)(5x – 2)

15x2 – 16x + 4 – (4x + 10)(2x + 5)(5x – 2)

the numerator.

3x – 2

2x + 5–

5x – 2

2 2x + 52x + 5

5x – 25x – 2

The LCD is (2x + 5)(5x – 2),

so multiply by

and by .

3x – 2 2x + 5

(5x – 2) (5x – 2)

2 5x – 2

(2x + 5) (2x + 5)

The expression is undefined at x = – and x = because these values of x make the factors (2x + 5) and (5x – 2) equal 0.

15x2 – 16x + 4 – 4x – 10(2x + 5)(5x – 2)

3x – 2

2x + 5–

5x – 2

2x2 + 64

x2 – 64–

x – 4

2x2 + 64 – (x – 4)(x – 8)(x – 8)(x + 8)

2x2 + 64 – (x2 – 12x + 32)(x – 8)(x + 8)

the numerator.

Factor the denominators.2x2 + 64

(x – 8)(x + 8)–

x – 4

The LCD is (x – 3)(x + 8),

so multiply by .x – 4 x + 8

(x – 8) (x – 8)

2x2 + 64

(x – 8)(x + 8)–

x – 4 x – 8x – 8

x2 + 12x + 32(x – 8)(x + 8)

standard form.

(x + 8)(x + 4)(x – 8)(x + 8) Factor the numerator.

x + 4x – 8 Divide out common factors.

The expression is undefined at x = 8 and x = –8 because these values of x make the factors (x + 8) and (x – 8) equal 0.

2x2 + 64

x2 – 64–

x – 4

2x2 + 64 – x2 + 12x – 32)(x – 8)(x + 8)

Some rational expressions are complex fractions. A complex fraction contains one or more fractions in its numerator, its denominator, or both. Examples of complex fractions are shown below.

Recall that the bar in a fraction represents division. Therefore, you can rewrite a complex fraction as a division problem and then simplify. You can also simplify complex fractions by using the LCD of the fractions in the numerator and denominator.

Example 5A: Simplifying Complex Fractions

Simplify. Assume that all expressions are defined.

Write as division.

x + 2x – 1x – 3x + 5

Write the complex fraction as division.

x + 2x – 1

x – 3x + 5

Multiply by the

reciprocal.x + 2x – 1

x + 5x – 3

Multiply.x2 + 7x + 10x2 – 4x + 3

(x + 2)(x + 5)(x – 1)(x – 3)

Example 5B: Simplifying Complex Fractions

The LCD is 2x.

Multiply the numerator and denominator of the complex fraction by the LCD of the fractions in the numerator and denominator.

x – 1x

+ (2x)(2x)

Divide out common

factors.

Simplify.x2 + 6

2(x – 1)

(3)(2) + (x)(x)(x – 1)(2)

Example 5B Continued

x2 + 62x – 2

Write as division.

x + 1x2 – 1

xx – 1

x + 1x2 – 1

xx – 1

Multiply by the reciprocal.x + 1x2 – 1

x – 1x

x + 1(x – 1)(x + 1)

x – 1x

Divide out common factors.

Factor the denominator.

Write as division.

20x – 1

63x – 3

20x – 1

63x – 3

Multiply by the reciprocal.20x – 1

3x – 36

Check It Out! Example 5b Continued

Divide out common factors.

20x – 1

3(x – 1)6

Check It Out! Example 5c

The LCD is (2x)(x – 2).

Multiply the numerator and denominator of the complex fraction by the LCD of the fractions in the numerator and denominator.

x + 4x – 2

(2x)(x – 2)

+ (2x)(x – 2)(2x)(x – 2)

Check It Out! Example 5c Continued

Divide out common

factors.

Simplify.3x – 6

(x + 4)(2x)

(2)(x – 2) + (x – 2)(x + 4)(2x)

3(x – 2)2x(x + 4)

Example 6: Transportation Application

A hiker averages 1.4 mi/h when walking downhill on a mountain trail and 0.8 mi/h on the return trip when walking uphill. What is the hiker’s average speed for the entire trip? Round to the nearest tenth.

Total distance: 2d Let d represent the

one-way distance.

+Total time: Use the formula t = . rd

2dAverage speed:The average speed is

. total timetotal distance

2d= = and = = .

1.4d d

5d0.8d d

2d(28)

(28) (28)

The LCD of the fractions in the

denominator is 28.

56d20d + 35d

Combine like terms and

divide out common factors.

Simplify.

55d55d

≈ 1.0

The hiker’s average speed is 1.0 mi/h.

Example 6 Continued

Justin’s average speed on his way to school is 40 mi/h, and his average speed on the way home is 45 mi/h. What is Justin’s average speed for the entire trip? Round to the nearest tenth.

Total distance: 2d Let d represent the

one-way distance.

+Total time: Use the formula t = . rd

2dAverage speed:The average speed is

. total timetotal distance

(360)d

2d(360)

The LCD of the fractions in the

denominator is 360.

720d9d + 8d

Combine like terms and

divide out common factors.

Simplify.

720d17d

≈ 42.4

Justin’s average speed is 42.4 mi/h.

Lesson Quiz: Part I

3x2 – 2x + 7(x – 2)(x + 1)

x ≠ –1, 2

x – 9x – 4

x ≠ 4, –4

1. 2x + 1x – 2

+x – 3x + 1

x x + 4

–x + 36x2 – 16

3. Find the least common multiple of x2 – 6x + 5and x2 + x – 2.

(x – 5)(x – 1)(x + 2)

Lesson Quiz: Part II

48 mi/h

5. Tyra averages 40 mi/h driving to the airportduring rush hour and 60 mi/h on the return triplate at night. What is Tyra’s average speed forthe entire trip?

4. Simplify . Assume that all expressions are defined.

x + 2x2 – 4

xx – 2

Adding and Subtracting Rational Expressions - Cobb … · Adding and Subtracting Rational...

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