## Basic Math & Pre-Algebra For Dummies, 2nd Edition (2014)

### Part I. Getting Started with Basic Math and Pre-Algebra

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*In this part…*

· See how the number system was invented and how it works

· Identify four important sets of numbers: counting numbers, integers, rational numbers, and real numbers

· Use place value to write numbers of any size

· Round numbers to make calculating quicker

· Work with the Big Four operations: adding, subtracting, multiplying, and dividing

### Chapter 1. Playing the Numbers Game

*In This Chapter*

Finding out how numbers were invented

Looking at a few familiar number sequences

Examining the number line

Understanding four important sets of numbers

One useful characteristic about numbers is that they're *conceptual,* which means that, in an important sense, they're all in your head. (This fact probably won't get you out of having to know about them, though — nice try!)

For example, you can picture three of anything: three cats, three baseballs, three cannibals, three planets. But just try to picture the concept of three all by itself, and you find it's impossible. Oh, sure, you can picture the numeral 3, but the *threeness* itself — much like love or beauty or honor — is beyond direct understanding. But when you understand the *concept* of three (or four, or a million), you have access to an incredibly powerful system for understanding the world: mathematics.

In this chapter, I give you a brief history of how numbers came into being. I discuss a few common *number sequences* and show you how these connect with simple math *operations* like addition, subtraction, multiplication, and division.

After that, I describe how some of these ideas come together with a simple yet powerful tool: the *number line.* I discuss how numbers are arranged on the number line, and I also show you how to use the number line as a calculator for simple arithmetic. Finally, I describe how the *counting numbers* (1, 2, 3, ...) sparked the invention of more unusual types of numbers, such as *negative numbers, fractions,* and *irrational numbers.* I also show you how these *sets of numbers* are *nested* — that is, how one set of numbers fits inside another, which fits inside another.

*Inventing Numbers*

Historians believe that the first number systems came into being at the same time as agriculture and commerce. Before that, people in prehistoric, hunter-gatherer societies were pretty much content to identify bunches of things as “a lot” or “a little.”

But as farming developed and trade between communities began, more precision was needed. So people began using stones, clay tokens, and similar objects to keep track of their goats, sheep, oil, grain, or whatever commodity they had. They exchanged these tokens for the objects they represented in a one-to-one exchange.

Eventually, traders realized that they could draw pictures instead of using tokens. Those pictures evolved into tally marks and, in time, into more complex systems. Whether they realized it or not, their attempts to keep track of commodities led these early humans to invent something entirely new: *numbers*.

Throughout the ages, the Babylonians, Egyptians, Greeks, Romans, Mayans, Arabs, and Chinese (to name just a few) all developed their own systems of writing numbers.

Although Roman numerals gained wide currency as the Roman Empire expanded throughout Europe and parts of Asia and Africa, the more advanced system that the Arabs invented turned out to be more useful. Our own number system, the Hindu–Arabic numbers (also called decimal numbers), is closely derived from these early Arabic numbers.

*Understanding Number Sequences*

Although humans invented numbers for counting commodities, as I explain in the preceding section, they soon put them to use in a wide range of applications. Numbers were useful for measuring distances, counting money, amassing an army, levying taxes, building pyramids, and lots more.

But beyond their many uses for understanding the external world, numbers have an internal order all their own. So numbers are not only an *invention,* but equally a *discovery:* a landscape that seems to exist independently, with its own structure, mysteries, and even perils.

One path into this new and often strange world is the *number sequence:* an arrangement of numbers according to a rule. In the following sections, I introduce you to a variety of number sequences that are useful for making sense of numbers.

*Evening the odds*

One of the first facts you probably heard about numbers is that all of them are either even or odd. For example, you can split an even number of marbles *evenly* into two equal piles. But when you try to divide an odd number of marbles the same way, you always have one *odd,* leftover marble. Here are the first few even numbers:

2 4 6 8 10 12 14 16 ...

You can easily keep the sequence of even numbers going as long as you like. Starting with the number 2, keep adding 2 to get the next number.

Similarly, here are the first few odd numbers:

1 3 5 7 9 11 13 15 ...

The sequence of odd numbers is just as simple to generate. Starting with the number 1, keep adding 2 to get the next number.

Patterns of even or odd numbers are the simplest number patterns around, which is why kids often figure out the difference between even and odd numbers soon after learning to count.

*Counting by threes, fours, fives, and so on*

When you get used to the concept of counting by numbers greater than 1, you can run with it. For example, here's what counting by threes, fours, and fives looks like:

Threes: 3 6 9 12 15 18 21 24 ...

Fours: 4 8 12 16 20 24 28 32 ...

Fives: 5 10 15 20 25 30 35 40 ...

Counting by a given number is a good way to begin learning the multiplication table for that number, especially for the numbers you're kind of sketchy on. (In general, people seem to have the most trouble multiplying by 7, but 8 and 9 are also unpopular.) In Chapter __3__, I show you a few tricks for memorizing the multiplication table once and for all.

These types of sequences are also useful for understanding factors and multiples, which you get a look at in Chapter __8__.

*Getting square with square numbers*

When you study math, sooner or later, you probably want to use visual aids to help you see what numbers are telling you. (Later in this book, I show you how one picture can be worth a thousand numbers when I discuss geometry in Chapter __16__ and graphing in Chapter __17__.)

The tastiest visual aids you'll ever find are those little square cheese-flavored crackers. (You probably have a box sitting somewhere in the pantry. If not, saltine crackers or any other square food works just as well.) Shake a bunch out of a box and place the little squares together to make bigger squares. Figure __1-1__ shows the first few.

*Illustration by Wiley, Composition Services Graphics*

**Figure 1-1:** Square numbers.

Voilà! The square numbers:

1 4 9 16 25 36 49 64 ...

You get a *square number* by multiplying a number by itself, so knowing the square numbers is another handy way to remember part of the multiplication table. Although you probably remember without help that 2 × 2 = 4 you may be sketchy on some of the higher numbers, such as 7 × 7 = 49. Knowing the square numbers gives you another way to etch that multiplication table forever into your brain, as I show you in Chapter __3__.

Square numbers are also a great first step on the way to understanding exponents, which I introduce later in this chapter and explain in more detail in Chapter __4__.

*Composing yourself with composite numbers*

Some numbers can be placed in rectangular patterns. Mathematicians probably should call numbers like these “rectangular numbers,” but instead they chose the term *composite numbers.* For example, 12 is a composite number because you can place 12 objects in rectangles of two different shapes, as in Figure __1-2__.

*Illustration by Wiley, Composition Services Graphics*

**Figure 1-2:** The number 12 laid out in two rectangular patterns.

As with square numbers, arranging numbers in visual patterns like this tells you something about how multiplication works. In this case, by counting the sides of both rectangles, you find out the following:

3 × 4 = 12

2 × 6 = 12

Similarly, other numbers such as 8 and 15 can also be arranged in rectangles, as in Figure __1-3__.

*Illustration by Wiley, Composition Services Graphics*

**Figure 1-3:** Composite numbers, such as 8 and 15, can form rectangles.

As you can see, both these numbers are quite happy being placed in boxes with at least two rows and two columns. And these visual patterns show this:

2 × 4 = 8

3 × 5 = 15

The word *composite* means that these numbers are *composed of* smaller numbers. For example, the number 15 is composed of 3 and 5 — that is, when you multiply these two smaller numbers, you get 15. Here are all the composite numbers from 1 to 16:

4 6 8 9 10 12 14 15 16

Notice that all the square numbers (see “Getting square with square numbers”) also count as composite numbers because you can arrange them in boxes with at least two rows and two columns. Additionally, a lot of other nonsquare numbers are also composite numbers.

*Stepping out of the box with prime numbers*

Some numbers are stubborn. Like certain people you may know, these numbers — called *prime numbers* — resist being placed in any sort of a box. Look at how Figure __1-4__ depicts the number 13, for example.

*Illustration by Wiley, Composition Services Graphics*

**Figure 1-4:** Unlucky 13, a prime example of a number that refuses to fit in a box.

Try as you may, you just can't make a rectangle out of 13 objects. (That fact may be one reason the number 13 got a bad reputation as unlucky.) Here are all the prime numbers less than 20:

2 3 5 7 11 13 17 19

As you can see, the list of prime numbers fills the gaps left by the composite numbers (see the preceding section). Therefore, every counting number is either prime or composite. The only exception is the number 1, which is neither prime nor composite. In Chapter __8__, I give you a lot more information about prime numbers and show you how to *decompose* a number — that is, break down a composite number into its prime factors.

*Multiplying quickly with exponents*

Here's an old question whose answer may surprise you: Suppose you took a job that paid you just 1 penny the first day, 2 pennies the second day, 4 pennies the third day, and so on, doubling the amount every day, like this:

1 2 4 8 16 32 64 128 256 512 ...

As you can see, in the first ten days of work, you would've earned a little more than $10 (actually, $10.23 — but who's counting?). How much would you earn in 30 days? Your answer may well be, “I wouldn't take a lousy job like that in the first place.” At first glance, this looks like a good answer, but here's a glimpse at your second ten days’ earnings:

... 1,024 2,048 4,096 8,192 16,384 32,768 65,536 131,072 262,144 524,288 ...

By the end of the second 10 days, your total earnings would be over $10,000. And by the end of 30 days, your earnings would top out around $10,000,000! How does this happen? Through the magic of exponents (also called *powers*). Each new number in the sequence is obtained by multiplying the previous number by 2:

As you can see, the notation 2^{4} means *multiply 2 by itself 4 times.*

You can use exponents on numbers other than 2. Here's another sequence you may be familiar with:

1 10 100 1,000 10,000 100,000 1,000,000 …

In this sequence, every number is 10 times greater than the number before it. You can also generate these numbers using exponents:

This sequence is important for defining *place value,* the basis of the decimal number system, which I discuss in Chapter __2__. It also shows up when I discuss decimals in Chapter __11__ and scientific notation in Chapter __15__. You find out more about exponents in Chapter __5__.

*Looking at the Number Line*

As kids outgrow counting on their fingers (and use them only when trying to remember the names of all seven dwarfs), teachers often substitute a picture of the first ten numbers in order, like the one in Figure __1-5__.

*Illustration by Wiley, Composition Services Graphics*

**Figure 1-5:** Basic number line.

This way of organizing numbers is called the *number line.* People often see their first number line — usually made of brightly colored construction paper — pasted above the blackboard in school. The basic number line provides a visual image of the *counting numbers* (also called the *natural numbers*), the numbers greater than 0. You can use it to show how numbers get bigger in one direction and smaller in the other.

In this section, I show you how to use the number line to understand a few basic but important ideas about numbers.

*Adding and subtracting on the number line*

You can use the number line to demonstrate simple addition and subtraction. These first steps in math become a lot more concrete with a visual aid. Here's the main point to remember:

· As you go *right*, the numbers go *up,* which is *addition* (+).

· As you go *left*, the numbers go *down,* which is *subtraction* (−).

For example, 2 + 3 means you *start at* 2 and *jump up* 3 spaces to 5, as Figure __1-6__ illustrates.

*Illustration by Wiley, Composition Services Graphics*

**Figure 1-6:** Moving through the number line from left to right.

As another example, 6 − 4 means *start* at 6 and *jump down* 4 spaces to 2. That is, 6 − 4 = 2. See Figure __1-7__.

*Illustration by Wiley, Composition Services Graphics*

**Figure 1-7:** Moving through the number line from right to left.

*Illustration by Wiley, Composition Services Graphics*

**Figure 1-8:** The number line starting at 0 and continuing with 1, 2, 3, ... 10.

You can use these simple up and down rules repeatedly to solve a longer string of added and subtracted numbers. For example, 3 + 1 − 2 + 4 − 3 − 2 means 3, *up* 1, *down* 2, *up* 4, *down* 3, and *down* 2. In this case, the number line shows you that 3 + 1 − 2 + 4 − 3 − 2 = 1.

I discuss addition and subtraction in greater detail in Chapter __3__.

*Getting a handle on nothing, or zero*

An important addition to the number line is the number 0, which means *nothing, zilch, nada.* Step back a moment and consider the bizarre concept of nothing. For one thing — as more than one philosopher has pointed out — by definition, *nothing* doesn't exist! Yet we routinely label it with the number 0, as in Figure __1-8__.

Actually, mathematicians have an even more precise labeling of *nothing* than zero. It's called the *empty* set, which is sort of the mathematical version of a box containing nothing. I introduce this concept, plus a little basic set theory, in Chapter __20__.

*Nothing* sure is a heavy trip to lay on little kids, but they don't seem to mind. They understand quickly that when you have three toy trucks and someone else takes away all three of them, you're left with zero trucks. That is, 3 − 3 = 0. Or, placing this on the number line, 3 − 3 means start at 3 and go down 3, as in Figure __1-9__.

*Illustration by Wiley, Composition Services Graphics*

**Figure 1-9:** Starting at 3 and moving down three.

In Chapter __2__, I show you the importance of 0 as a *placeholder* in numbers and discuss how you can attach *leading zeros* to a number without changing its value.

**Infinity: Imagining a never-ending story**

The arrows at the ends of the number line point onward to a place called *infinity,* which isn't really a place at all — just the idea of *foreverness* because the numbers go on forever. But what about a million billion trillion quadrillion — do the numbers go even higher than that? The answer is yes, because for any number you name, you can add 1 to it.

The wacky symbol ∞ represents infinity. Remember, though, that ∞ isn't really a number but the *idea* that the numbers go on forever.

Because ∞ isn't a number, you can't technically add the number 1 to it, any more than you can add the number 1 to a cup of coffee or your Aunt Louise. But even if you could, ∞ + 1 would equal ∞.

*Taking a negative turn: Negative numbers*

When people first find out about subtraction, they often hear that you can't take away more than you have. For example, if you have four pencils, you can take away one, two, three, or even all four of them, but you can't take away more than that.

It isn't long, though, before you find out what any credit card holder knows only too well: You can, indeed, take away more than you have — the result is a *negative number.* For example, if you have $4 and you owe your friend $7, you're $3 in debt. That is, 4 − 7 = −3. The minus sign in front of the 3 means that the number of dollars you have is three less than 0. Figure __1-10__ shows how you place negative whole numbers on the number line.

*Illustration by Wiley, Composition Services Graphics*

**Figure 1-10:** Negative whole numbers on the number line.

Adding and subtracting on the number line works pretty much the same with negative numbers as with positive numbers. For example, Figure __1-11__ shows how to subtract 4 − 7 on the number line.

*Illustration by Wiley, Composition Services Graphics*

**Figure 1-11:** Subtracting 4 – 7 on the number line.

You find out all about working with negative numbers in Chapter __4__.

Placing 0 and the negative counting numbers on the number line expands the set of counting numbers to the set of *integers*. I discuss the integers in further detail later in this chapter.

*Multiplying the possibilities*

Suppose you start at 0 and circle every other number on a number line, as in Figure __1-12__. As you can see, all the even numbers are now circled. In other words, you've circled all the *multiples of two.* (You can find out more about multiples in Chapter __8__.) You can now use this number line to multiply any number by two. For example, suppose you want to multiply 5 × 2. Just start at 0 and jump five circled spaces to the right.

*Illustration by Wiley, Composition Services Graphics*

**Figure 1-12:** Multiplying 5 × 2 using the number line.

This number line shows you that 5 × 2 = 10.

Similarly, to multiply −3 × 2, start at 0 and jump three circled spaces to the left (that is, in the negative direction). Figure __1-13__ shows you that −3 × 2 = −6. What's more, you can now see why multiplying a negative number by a positive number always gives you a negative result. (I talk about multiplying by negative numbers in Chapter __4__.)

*Illustration by Wiley, Composition Services Graphics*

**Figure 1-13:** 3 × 2 = –6 as depicted on the number line.

*Dividing things up*

You can also use the number line to divide. For example, suppose you want to divide 6 by some other number. First, draw a number line that begins at 0 and ends at 6, as in Figure __1-14__.

*Illustration by Wiley, Composition Services Graphics*

**Figure 1-14:** Number line from 0 to 6.

Now, to find the answer to 6 ÷ 2, just split this number line into two equal parts, as in Figure __1-15__. This split (or *division*) occurs at 3, showing you that 6 ÷ 2 = 3.

__Illustration by Wiley, Composition Services Graphics__

**Figure 1-15:** Getting the answer to 6 ÷ 2 by splitting the number line.

Similarly, to divide 6 ÷ 3, split the same number line into three equal parts, as in Figure __1-16__. This time you have two splits, so use the one closest to 0. This number line shows you that 6 ÷ 3 = 2.

*Illustration by Wiley, Composition Services Graphics*

**Figure 1-16:** Dividing 6 ÷ 3 with the number line.

But suppose you want to use the number line to divide a small number by a larger number. For example, maybe you want to know the answer to 3 ÷ 4. Following the method I show you earlier, first draw a number line from 0 to 3. Then split it into four equal parts. Unfortunately, none of these splits has landed on a number. It's not a mistake — you just have to add some new numbers to the number line, as you can see in Figure __1-17__.

*Illustration by Wiley, Composition Services Graphics*

**Figure 1-17:** Fractions on the number line.

Welcome to the world of *fractions.* With the number line labeled properly, you can see that the split closest to 0 is . This image tells you that . The similarity of the expression 3 ÷ 4 and the fraction is no accident. Division and fractions are closely related. When you divide, you cut things up into equal parts, and fractions are often the result of this process. (I explain the connection between division and fractions in more detail in Chapters __9__ and __10__.)

*Discovering the space in between: Fractions*

Fractions help you fill in a lot of the spaces on the number line that fall between the counting numbers. For example, Figure __1-18__ shows a close-up of a number line from 0 to 1.

*Illustration by Wiley, Composition Services Graphics*

**Figure 1-18:** Number line depicting some fractions from 0 to 1.

This number line may remind you of a ruler or a tape measure, with a lot of tiny fractions filled in. In fact, rulers and tape measures really are portable number lines that allow carpenters, engineers, and savvy do-it-yourselfers to measure the length of objects with precision.

Adding fractions to the number line expands the set of integers to the set of *rational numbers.* I discuss the rational numbers in greater detail in Chapter __25__.

In fact, no matter how small things get in the real world, you can always find a tiny fraction to approximate it as closely as you need. Between any two fractions on the number line, you can always find another fraction. Mathematicians call this trait the *density* of fractions on the real number line, and this type of density is a topic in a very advanced area of math called *real analysis.*

*Four Important Sets of Numbers*

In the preceding section, you see how the number line grows in both the positive and negative directions and fills in with a lot of numbers in between. In this section, I provide a quick tour of how numbers fit together as a set of nested systems, one inside the other.

When I talk about a set of numbers, I'm really just talking about a group of numbers. You can use the number line to deal with four important sets of numbers:

· **Counting numbers (also called natural numbers):** The set of numbers beginning 1, 2, 3, 4 ... and going on infinitely

· **Integers:** The set of counting numbers, zero, and negative counting numbers

· **Rational numbers:** The set of integers and fractions

· **Real numbers:** The set of rational and irrational numbers

The sets of counting numbers, integers, rational, and real numbers are nested, one inside another. This nesting of one set inside another is similar to the way that a city (for example, Boston) is inside a state (Massachusetts), which is inside a country (the United States), which is inside a continent (North America). The set of counting numbers is inside the set of integers, which is inside the set of rational numbers, which is inside the set of real numbers.

*Counting on the counting numbers*

The set of *counting numbers* is the set of numbers you first count with, starting with 1. Because they seem to arise naturally from observing the world, they're also called the *natural numbers:*

1 2 3 4 5 6 7 8 9 ...

The counting numbers are infinite, which means they go on forever.

When you add two counting numbers, the answer is always another counting number. Similarly, when you multiply two counting numbers, the answer is always a counting number. Another way of saying this is that the set of counting numbers is *closed* under both addition and multiplication.

*Introducing integers*

The set of *integers* arises when you try to subtract a larger number from a smaller one. For example, 4 − 6 = −2. The set of integers includes the following:

· The counting numbers

· Zero

· The negative counting numbers

Here's a partial list of the integers:

... −4 −3 −2 −1 0 1 2 3 4 ...

Like the counting numbers, the integers are closed under addition and multiplication. Similarly, when you subtract one integer from another, the answer is always an integer. That is, the integers are also closed under subtraction.

*Staying rational*

Here's the set of *rational numbers:*

· Integers

· Counting numbers

· Zero

· Negative counting numbers

· Fractions

Like the integers, the rational numbers are closed under addition, subtraction, and multiplication. Furthermore, when you divide one rational number by another, the answer is always a rational number. Another way to say this is that the rational numbers are closed under division.

*Getting real*

Even if you filled in all the rational numbers, you'd still have points left unlabeled on the number line. These points are the irrational numbers.

An *irrational number* is a number that's neither a whole number nor a fraction. In fact, an irrational number can only be approximated as a *non-repeating decimal.* In other words, no matter how many decimal places you write down, you can always write down more; furthermore, the digits in this decimal never become repetitive or fall into any pattern. (For more on repeating decimals, see Chapter __11__.)

The most famous irrational number is π (you find out more about π when I discuss the geometry of circles in Chapter __17__):

Together, the rational and irrational numbers make up the *real numbers,* which comprise every point on the number line. In this book, I don't spend too much time on irrational numbers, but just remember that they're there for future reference.