Algebra I. Lesson 2

Instruction 2-1

 Pre-Test Post-Test Q&A Q&A

Integers and the Number Line | Adding and Subtracting Integers | Inequalities and the Number Line | Comparing and Ordering Rational Numbers | Dividing Rational Numbers

Integers and the Number Line

We use natural numbers or counting numbers to count objects or express quantities. For example the set of numbers 1, 2, 3, 4....  are natural numbers.

Let's look at the example below. Table 2.1shows some amounts of money. These are the amounts John earned during six days.

Date Amount of Money and Its Source
4-1-2003 \$120 from saving account
4-2-2003 \$230 from selling items
4-3-2003 \$760 from tax return
4-4-2003  \$120 from selling items
4-5-2003 \$240 refund from returned items
4-6-2003 \$82 from selling items
Table 2.1

All the amounts in Table 2.1 are gained. Nothing is recorded as losses. You can make these records simpler. Place them in a table such as Table 2.2. Both tables show the same information. But the second one is much simpler.

4-1-2003 \$120
4-2-2003 \$230
4-3-2003  \$760
4-4-2003  \$120
4-5-2003  \$240
4-6-2003 \$82
Table 2.2

As you see, we used simply whole numbers in this case. There are some cases in which one does not gain money only. He both gains and loses. For example, look at Table 2.3. This table shows the operations on a checking account over six days.

Date Operation
5-1-2003 \$120 Withdrawal
5-2-2003 \$230 Direct Deposit
5-3-2003 \$760 Direct Deposit
5-4-2003  \$120 Electronic Debit
5-5-2003 \$240 Electronic Deposit
5-6-2003 \$82 Overdraft Charge

Table 2.3

We can not use only the whole numbers to represent the data in Table 2.3. We must use another set of numbers. They are called integers.

Let's recall natural numbers. They are 0, 1, 2, 3, … In this set, 0 is less than 1, 1 is less than 2, 2 is less than 3, and so forth. So, 0 is the smallest number in this set. Is there any number less than zero? Yes.

There are many numbers less than 0. These numbers called negative numbers. Such numbers are used to represent quantities such as losses, temperatures below zero, debts, withdrawals, or altitudes below the sea level.

If we show \$120 deposit by +\$120, we can show \$240 withdrawal by –\$240. If the depth of a valley is 32 feet below the sea level, then we can show its altitude by –32 ft. Negative numbers along with natural numbers form the set of integers.

Set of numbers Types of numbers included in the set
Whole numbers

(1,2,3,4, . . .)

Natural numbers (0,1,2,3, . . .)
Integers (. . . -3,-2,-1,0,1,2,3, . . .)

Practice 1. Use labels natural number, whole number, integer for each number.

(a) –32 (b) 45 (c) –76 (d) 92

(e) 0

Practice 2. Which numbers are integers?

–3, 4.5, 32, –23, 65, 15, –43.6

Real Life Application 1. Show each measure by an integer.

(a) altitude of a building 120 feet above the sea level

(b) altitude of a submarine 860 feet below the sea level

(c) 12˚ F below zero

(d) altitude of the bottom of a valley whose depth is 110 feet below the sea level

(e) 32˚ F above zero

(f) \$328 withdrawal from a saving account

To understand the integers, it is a good idea to use a diagram. This is called a number line. To graph a number line, first draw a line with an arrow on each end. Beyond the right hand arrow, put a plus. Beyond the left hand arrow, put a minus. (Figure 2.4).

Figure 2.4

Find the midpoint of this number line. Label it as 0. Choose a length on the line as one unit. Start from 0 and mark the line in both directions. The distance between each mark and the one before it or the one after it is one unit  (Figure 2.5).

Figure 2.5

Label the marks from 0 to the right as 1, 2, 3, …. Label the marks from 0 to the left as –1, –2, –3, … (Figure 2.6). This is a display of integers on a number line.

We will use this line often in problems and other topics. Each number is called the coordinate of the point. Zero is neither positive nor negative. It is used as a reference for splitting numbers into two signed parts.

Figure 2.6

A number written using one of the signs – or + called a signed number. Sometimes, we drop the + signs of positive numbers. So, any whole number is also a positive numbers.

Practice 3. Graph the members of each set on a number line.

A = {–4, –3, 0, 2, 5}

B = {–6, –3, 1, 2, 4}

C = {–5, –1, 1, 3, 5}

Solution.

Set A

Set B

Set C

Real Life Application 2. Write integers that can be used to the real life situations below.

(a) a temperature of 15˚ F below zero

(b) a deposit of \$340

(c) a withdrawal of \$120

(d) a gain of \$320

(e) a debt of \$86

(f) 120 feet above the sea level

(g) 82 feet below the sea level

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Now let's do Practice Exercise 2-1 (top).

Next Page: Adding and Subtracting Integers (top)