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4.3 Adding Integers, Part II • 215

Opposites are all around us. If you move forward two spaces in a board game

and then move back in the opposite direction two spaces, you’re back where you

started. In tug-of-war, if one team pulling on the rope pulls exactly as hard as the

team on the opposite side, no one moves. If an element has the same number of

positively charged protons as it does of negatively charged electrons, then the

element has no charge.

In what ways have you worked with opposites in mathematics?

Learning GoalsIn this lesson, you will:

Model the addition of integers using two-color

counters.

Develop a rule for adding integers.

Two-ColorCountersAdding Integers, Part II

Key Term additive inverses

216 • Chapter 4 Addition and Subtraction with Rational Numbers

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Problem 1 Two-Color Counters

1. Use the number line model to determine each sum.

a. 3 1 (23) 5

–15 –10 –5 0 5 10 15

b. (214) 1 14 5

–15 –10 –5 0 5 10 15

c. 8 1 (28) 5

–15 –10 –5 0 5 10 15

d. What pattern do you notice?

Two numbers with the sum of zero are called additiveinverses.

Addition of integers can also be modeled using two-color counters that represent

positive (1) charges and negative (2) charges. One color, usually red, represents the negative

number, or negative charge. The other color, usually yellow, represents the positive number,

or positive charge. In this book, gray shading will represent the negative number, and no

shading will represent the positive number.

– 5 21 + 5 11

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4.3 Adding Integers, Part II • 217

2. What is the value of each – and + pair shown in the second model?

3. Describe how you can change the numbers of – and + counters in the model, but

leave the sum unchanged.

You can model the expression 3 1 (23) in different ways using

two-color counters:

(–3) +3

+

+

+

–

–

–

Three positive charges and

three negative charges

have no charge.

3 1 (23) 5 0

(–3) +3

+–

+–

+–

Each positive charge is paired

with a negative charge.

3 1 (23) 5 0

218 • Chapter 4 Addition and Subtraction with Rational Numbers

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Example1: 5 1 8

+ +

+ +

+ +

+ +

+ +

+ +

+

There are 13 positive charges in the model. The sum is 13.

Example2: 5 1 (28)

+ +

+ +

+

– –

– –

– –

– –

+ +

+ +

+

– –

– –

– –

– –

There are five

+ – pairs.

The value of those

pairs is 0.

There are 3 – ,

or negative

charges, remaining.

There are 3 negative charges remaining. The sum of 5 1 (28) is 23.

4. Create another model to represent a sum of 23. Write the appropriate number sentence.

Let’s consider two examples where integers are added using two-color counters.

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4.3 Adding Integers, Part II • 219

5. Share your model with your classmates. How are they the same? How are they different?

6. Write a number sentence to represent each model.

a.

+

+

––

––

––

b.

++

++

++

+

–

––

c.

++

+

+

++

– –

––

– –

––

d. +

+

++

++

+

– –

––

– –

e.

+

+

++

– –

––

f.

– ––

–

––––

220 • Chapter 4 Addition and Subtraction with Rational Numbers

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7. Does the order in which you wrote the integers in your number sentence matter?

How do you know?

8. Write each number sentence in Question 6 a second way.

9. Draw a model for each, and then complete the number sentence.

a. 29 1 (24) 5 b. 29 1 4 5

c. 9 1 (24) 5 d. 9 1 4 5

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10. Complete the model to determine the unknown integer.

a. 1 1 5 24 b. 23 1 5 7

+

––

–

c. 7 1 5 21

+ +

++

++

+

11. Describe the set of integers that makes each

sentence true.

a. What integer(s) when added to 27 give a

sum greater than 0?

b. What integer(s) when added to 27 give a sum of less than 0?

c. What integer(s) when added to 27 give a sum of 0?

4.3 Adding Integers, Part II • 221

Consider drawing a number line model or a two-color counter model

to help you answer each question.

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222 • Chapter 4 Addition and Subtraction with Rational Numbers

12. When adding two integers, what will the sign of the sum be if:

a. both integers are positive?

b. both integers are negative?

c. one integer is negative and one integer is

positive?

13. Write a rule that states how to determine the sum of any two

integers that have the same sign.

14. Write a rule that states how to determine the sum of any two

integers that have opposite signs.

What happens when you add a

negative and a positive integer and they both

have the same absolute value?

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4.3 Adding Integers, Part II • 223

15. Use your rule to determine each sum.

a. 258 1 (24) 5 b. 235 1 (215) 5

c. 233 1 (212) 5 d. 248 1 60 5

e. 26 1 (213) 5 f. 267 1 67 5

g. 105 1 (225) 5 h. 153 1 (237) 5

16. Determine each unknown addend.

a. 1 (225) 5 34 b. 1 26 5 12

c. 8 1 5 224 d. 212 1 5 224

e. 215 1 5 228 f. 1 18 5 23

Talk the Talk

Represent the sum of additive inverses in the graphic organizer provided. First, write a

number sentence. Then, represent your number sentence in words, using a number line

model, and using a two-color counter model.

Be prepared to share your solutions and methods.

224 • Chapter 4 Addition and Subtraction with Rational Numbers

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InWords

Two-ColorCounterModel

NumberSentence

NumberLineModel

AdditiveInversesandZero,0