The Rational Numbers
The rational numbers are ratios of integers (where the divisor is non-zero), such as 3/4, -2/17, etc. Note that the integers are included among the rational numbers, for example, the integer 3 can be written as 3/1, or even 6/2 .
The result of adding, subtracting, multiplying, or dividing rational numbers (so long as we don’t divide by zero) is another rational number. We say that the set of rational numbers is closed under addition, subtraction, multiplication, and division.
Of course, after extending the integers to the rational numbers, we again need to define what we mean by the sum, difference, product, and ratio of two rational numbers. This is discussed in detail on the page on fractions.
The Real Numbers
It turns out that the rational numbers are not enough to describe the world. Consider, for example, a right triangle whose two short sides are each 1 foot long. By the Pythagorean Theorem, the long side, the hypotenuse, has a length whose square equals 2. That length is referred to as the square root of 2.
It can be shown that there is no rational number whose square equals 2. Hence the number system needs to be extended once more. For our purposes a real number is a decimal expression whose digits may or may not terminate or repeat. It can also be shown that a real number is rational if and only if its digit repeat or terminate. Real numbers that aren’t rational are irrational
Hierarchy of Numbers
One of the harder things to understand and appreciate about numbers is that they form a hierarchy. Just like all trout are fish, all fish are vertebrates, and all vertebrates are animals, all natural numbers are integers, all integers are rational numbers, and all rational numbers are real numbers. On the other hand, just like some fish aren’t trout, some real numbers aren’t rational, some rational numbers aren’t integer, and some integers aren’t natural numbers. For some reason this tends to be very confusing, but it’s just like saying that all women are people, but some people aren’t women. (The same goes for men, of course, and then there are kids too.) Never mind the people, but you need to understand this language, and be prepared to give examples of the various types of numbers. The hierarchy of the number system is illustrated in this Figure:
Each number set contains the number sets it surrounds. For example the set of rational numbers contains all natural numbers (and all integers). The Figure also indicates which operations are possible in each set. For example, we can add, subtract, and multiply integers, and the result will be an integer. (The result of dividing two integers is not always an integer, for example 5/2 is not.) The examples given are in the set shown, but not in a smaller set. For example, 2/3 is a rational number. It’s also a real number, but it’s not an integer, and it’s not a natural number.
Later in this course we will learn about a yet more general set of numbers, the complex numbers.