# 7: The Properties of Real Numbers

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A quilt is formed by sewing many different pieces of fabric together. The pieces can vary in color, size, and shape. The combinations of different kinds of pieces provide for an endless possibility of patterns. Much like the pieces of fabric, mathematicians distinguish among different types of numbers. The kinds of numbers in an expression provide for an endless possibility of outcomes. We have already described counting numbers, whole numbers, and integers. In this chapter, we will learn about other types of numbers and their properties.

• 7.1: Rational and Irrational Numbers
A rational number is a number that can be written in the form p/q, where p and q are integers and q ≠ 0. Rational numbers consist of many decimals and all fractions and integers, both positive and negative. An irrational number is a number that cannot be written as the ratio of two integers. Its decimal form neither stops nor repeats. Some irrational numbers include pi and the square roots of numbers that are not perfect squares. Real numbers are numbers that are either rational or irrational.
• 7.2: Commutative and Associative Properties (Part 1)
The commutative properties have to do with order. If you change the order of the numbers when adding or multiplying, the result is the same. When adding or multiplying three numbers, changing the grouping of the numbers does not change the result. This is known as the Associative Property of Addition and Multiplication, respectively. So, addition and multiplication are commutative and associative. But, subtraction and division are neither commutative nor associative.
• 7.3: Commutative and Associative Properties (Part 2)
When we have to simplify algebraic expressions, we can often make the work easier by applying the Commutative or Associative Property first instead of automatically following the order of operations. No matter what you are doing, it is always a good idea to think ahead. When simplifying an expression, think about what your steps will be. For example, when adding and subtracting three or more terms involving decimals, look for terms that combine to give whole numbers.
• 7.4: Distributive Property
The Distributive Property states that if a, b, c are real numbers, then a(b + c) = ab + ac. In algebra, we use the Distributive Property to remove parentheses as we simplify expressions. When you distribute a negative number, you need to be extra careful to get the signs correct. Sometimes we need to use the Distributive Property as part of the order of operations.
• 7.5: Properties of Identity, Inverses, and Zero
Adding zero to any number doesn’t change the value. For this reason, we call 0 the additive identity. The opposite of a number is its additive inverse. The reciprocal of a number is its multiplicative inverse. A number and its reciprocal multiply to 1, which is the multiplicative identity. The product of any real number and 0 is 0. Zero divided by any real number except zero is zero. But division by zero is undefined.
• 7.6: Systems of Measurement (Part 1)
In this section we will see how to convert among different types of units, such as feet to miles or kilograms to pounds. The basic idea in all of the unit conversions will be to use a form of 1, the multiplicative identity, to change the units but not the value of a quantity.
• 7.7: Systems of Measurement (Part 2)
Performing arithmetic operations on measurements with mixed units of measures in the metric system requires the same care we used in the U.S. system. Many measurements in the United States are made in metric units. We make conversions between the systems just as we do within the systems—by multiplying by unit conversion factors. The U.S. and metric systems use different scales to measure temperature. The U.S. system uses degrees Fahrenheit. The metric system uses degrees Celsius.
• 7.8: The Properties of Real Numbers (Exercises)
• 7.9: The Properties of Real Numbers (Summary)

Figure 7.1 - Quiltmakers know that by rearranging the same basic blocks the resulting quilts can look very different. What happens when we rearrange the numbers in an expression? Does the resulting value change? We will answer these questions in this chapter as we will learn about the properties of numbers. (credit: Hans, Public Domain)

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