5.1 Algebraic Expressions

A young man and a young woman are standing next to each other wearing graduation caps and gowns. — **Figure 5.2** Two college graduates!Two college graduates! (credit: modification of work UC Davis College of Engineering/Flickr, CC BY 2.0)

Learning Objectives

After completing this section, you should be able to:

Convert between written and symbolic algebraic expressions and equations.
Simplify and evaluate algebraic expressions.
Add and subtract algebraic expressions.
Multiply and divide algebraic expressions.

Algebraic expressions are the building blocks of algebra. While a numerical expression (also known as an arithmetic expression) like $5 plus 3$ can represent only a single number, an algebraic expression such as $5 x plus 3$ can represent many different numbers. This section will introduce you to algebraic expressions, how to create them, simplify them, and perform arithmetic operations on them.

Algebraic Expressions and Equations

Xavier and Yasenia have the same birthday, but they were born in different years. This year Xavier is 20 years old and Yasenia is 23, so Yasenia is three years older than Xavier. When Xavier was 15, Yasenia was 18. When Xavier will be 33, Yasenia will be 36. No matter what Xavier’s age is, Yasenia’s age will always be 3 years more.

In the language of algebra, we say that Xavier's age and Yasenia's age are variable and the 3 is a constant. The ages change, or vary, so age is a variable. The 3 years between them always stays the same or has the same value, so the age difference is the constant. In algebra, letters of the alphabet are used to represent variables. The letters most often used for variables are $x$ , $y$ , $z$ , $a$ , $b$ , and $c$ . Suppose we call Xavier's age $x$ . Then we could use $x plus 3$ to represent Yasenia's age, as shown in the table below.

Xavier’s Age	Yasenia’s Age
15	18
20	23
33	36
$x$	$x plus 3$

To write algebraically, we need some symbols as well as numbers and variables. The symbols for the four basic arithmetic operations: addition, subtraction, multiplication, and division are summarized in Table 5.1, along with words we use for the operations and the result.

**Table 5.1**
Operation	Notation	Say:	The result is…
Addition	$a + b$	$a$ plus $b$	The sum of $a$ and $b$
Subtraction	$a$ − $b$	$a$ minus $b$	The difference of $a$ and $b$
Multiplication	$a$ ⋅ $b$ , ( $a$ )( $b$ ), ( $a$ ) $b$ , $a$ ( $b$ ), $ab$ , $ba$	$a$ times $b$	The product of $a$ and $b$
Division	$a$ ÷ $b$ , $a$ / $b$	$a$ divided by $b$	The quotient of $a$ and $b$

We perform these operations on two numbers. When translating from symbolic form to words, or from words to symbolic form, pay attention to the words of or and to help you find the numbers.

The sum of 5 and 3 means add 5 plus 3, which we write as $5 plus 3$ .
The difference of 9 and 2 means subtract 9 minus 2, which we write as $9 minus 2$ .
The product of 4 and 8 means multiply 4 times 8, which we can write as $4 times 8$ .
The quotient of 20 and 5 means divide 20 by 5, which we can write as $20 divided by 5$ .

What is the difference in English between a phrase and a sentence? A phrase expresses a single thought that is incomplete by itself, but a sentence makes a complete statement. “Running very fast” is a phrase, but “The football player was running very fast” is a sentence. A sentence has a subject and a verb. In algebra, we have expressions and equations. Example 1 and Example 2 used expressions. An expression is like an English phrase. Notice that the English phrases do not form a complete sentence because the phrase does not have a verb. The following table has examples of expressions, which are numbers, variables, or combinations of numbers and variables using operation symbols.

Expression	Words	English Phrase
$3 plus 5$	3 plus 5	The sum of three and five
$n minus 1$	$n$ minus one	The difference of $n$ and one
$6 times 7$	6 times 7	The product of six and seven
$x \div y$	$x$ divided by $y$	The quotient of $x$ and $y$

An equation is two expressions linked with an equal sign (the symbol =). When two quantities have the same value, we say they are equal and connect them with an equal sign. When you read the words the symbols represent in an equation, you have a complete sentence in English. The equal sign gives the verb. So, $a = b$ is read “ $a$ is equal to $b$ .” The following table has some examples of equations.

Equation	English Sentence
$3 + 5 = 8$	The sum of three and five is equal to eight.
$n - 1 = 14$	$n$ minus one equals fourteen.
$6 \cdot 7 = 42$	The product of six and seven is equal to forty-two.
$x equals 53$	$x$ is equal to fifty-three.
$y + 9 = 2 y - 3$	$y$ plus nine is equal to two times $y$ minus three.

Who Knew?

The Use of Variables

French philosopher and mathematician René Descartes (1596–1650) is usually given credit for the use of the letters $x$ , $y$ , and $z$ to represent unknown quantities in algebra. He introduced these ideas in his publication of La Geometrie, which was printed in 1637. In this publication, he also used the letters $a$ , $b$ , and $c$ to represent known quantities. There is a (possibly fictitious) story that, when the book was being printed for the first time, the printer began to run short of the last three letters of the alphabet. So the printer asked Descartes if it mattered which of $x$ , $y$ , or $z$ were used for the mathematical equations in the book. Descartes decided it made no difference to him; so the printer decided to use $x$ predominantly for the mathematics in the book, because the letters $y$ and $z$ would occur more often in the body of the text (written in French) than the letter $x$ would! This might explain why the letter $x$ is still used today as the most common variable to represent unknown quantities in algebra.

Simplifying and Evaluating Algebraic Expressions

To simplify an expression means to do all the math possible. For example, to simplify $4 \cdot 2 + 1$ we would first multiply $4 times 2$ to get 8 and then add 1 to get 9. We have introduced most of the symbols and notation used in algebra, but now we need to clarify the order of operations. Otherwise, expressions may have different meanings, and they may result in different values. Consider $2 + 7 \cdot 3$ . Do you add first or multiply first? Do you get different answers?

Add first:

9 \cdot 3 = 27

Multiply first:

2 + 21 = 23

Which one is correct?

Early on, mathematicians realized the need to establish some guidelines when performing arithmetic operations to ensure that everyone would get the same answer. Those guidelines are called the order of operations and are listed in the table below.

Step 1: Parentheses and Other Grouping Symbols	Simplify all expressions inside the parentheses or other grouping symbols, working on the innermost parentheses first.
Step 2: Exponents	Simplify all expressions with exponents.
Step 3: Multiplication and Division	Perform all multiplication and division in order from left to right. These operations have equal priority.
Step 4: Addition and Subtraction	Perform all addition and subtraction in order from left to right. These operations have equal priority.

In the last example, we simplified expressions using the order of operations. Now we'll evaluate some expressions—again following the order of operations. To evaluate an expression means to find the value of the expression when the variable is replaced by a given number.

Operations of Algebraic Expressions

Algebraic expressions are made up of terms. A term is a constant or the product of a constant and one or more variables. Examples of terms are 7, $y$ , 5 $x^{2}$ , 9 $a$ , and $b to the power 5$ . The constant that multiplies the variable is called the coefficient. Think of the coefficient as the number in front of the variable. Consider the algebraic expressions 5 $x^{2}$ , which has a coefficient of 5, and 9 $a$ , which has a coefficient of 9. If there is no number listed in front of the variable, then the coefficient is 1 since $x = 1 \cdot x$ .

Some terms share common traits. When two terms are constants or have the same variable and exponent, we say they are like terms. If there are like terms in an expression, you can simplify the expression by combining the like terms. We add the coefficients and keep the same variable.

Before looking at multiplying algebraic expressions we look at the Distributive Property, which says that to multiply a sum, first you multiply each term in the sum and then you add the products. For example, $5 (4 + 3) = 5 (4) + 5 (3) = 20 + 15 = 35$ can also be solved as $5 (4 + 3) = 5 (7) = 35$ . If we use a variable, then $5 (x + 3) = 5 x + 15$ .

We can extended this example to $(5 + 2) (4 + 3) = (5) (4) + (5) (3) + (2) (4) + (2) (3) = 20 + 15 + 8 + 6 = 49$ , which can also be solved as $(5 + 2) (4 + 3) = (7) (7) = 49$ . If we use variables, then $(x + 5) (x + 4) = (x) (x) + (x) (4) + (5) (x) + (5) (4) = x^{2} + 4 x + 5 x + 20 = x^{2} + 9 x + 20$ .

People in Mathematics

Al-Khwarizmi

Figure 5.3 Al-Khwarizmi

Abu Ja’far Muhammad ibn Musa Al-Khwarizmi was born around 780 AD, probably in or around the region of Khwarizm, which is now part of modern-day Uzbekistan. For most of his adult life, he worked as a scholar at the House of Wisdom in Baghdad, Iraq. He wrote many mathematical works during his life, but is probably most famous for his book Al-kitab al-muhtasar fi hisab al-jabr w’al’muqabalah, which translates to The Condensed Book on the Calculation of al-Jabr (completion) and al’muqabalah (balancing). The word al-jabr would eventually become the word we use to describe the topic that he was writing about in this book: algebra. From another book of his, with the Latin title Algoritmi de numero Indorum (Al-Khwarizmi on the Hindu Art of Reckoning ), our word algorithm is derived. In addition to writing on mathematics, Al-Khwarizmi wrote works on astronomy, geography, the sundial, and the calendar.

In 2012, Andrew Hacker wrote an opinion piece in the New York Times Magazine suggesting that teaching algebra in high school was a waste of time. Keith Devlin, a British mathematician, was asked to comment on Hacker's article by his students in his Stanford University Continuing Studies course "Mathematics: Making the Invisible Visible" on iTunes University. Devlin concludes that Hacker was displaying his ignorance of what algebra is.

Key Terms

variable
constant
expression
equation
equal sign
term
coefficient
like terms
Distributive Property

Key Concepts

Algebra is useful because it allows us to understand many situations in real life by modeling them with expressions.
Algebraic expressions are the building blocks of algebra. From algebraic expressions we can create algebraic equations.
Algebraic expressions are often simplified and evaluated using the four arithmetic operations.

Videos

Q&A: Why We Teach Algebra ↗ new tab

Formulas

Distributive Property: $a (b + c) = a b + a c$

Adapted from Contemporary Mathematics by OpenStax (openstax.org), licensed under CC BY-NC-SA 4.0. Changes were made. License: CC-BY-NC-SA-4.0.