Why use the order of operations?

What is [latex]3+5•\times2[/latex]? Is it 13 or 16? This may seem like a trick question, but there is actually only one correct answer.

Many years ago, mathematicians developed a standard order of operations that tells you which calculations to make first in an expression with more than one operation. In other words, order of operations simply refers to the specific order of steps you should follow when you solve a math expression. Without a standard procedure for making calculations, two people could get two different answers to the same problem, like the one above. So which is it, 13 or 16? By the end of this module you’ll know!

Some important terminology before we begin:

• operations/operators: In mathematics we call things like multiplication, division, addition, and subtraction operations.  They are the verbs of the math world, doing work on numbers and variables. The symbols used to denote operations are called operators, such as [latex]+{, }-{, }\times{, }\div[/latex]. As you learn more math, you will learn more operators.
• term: Examples of terms would be [latex]2x[/latex] and [latex]-\frac{3}{2}[/latex] or [latex]a^3[/latex]. Even lone integers can be a term, like 0.
• expression: A mathematical expression is one that connects terms with mathematical operators. For example  [latex]\frac{1}{2}+\left(2^2\right)- 9\div\frac{6}{7}[/latex] is an expression.

Combining Like Terms

One way we can simplify expressions is to combine like terms. Like terms are terms where the variables match exactly (exponents included). Examples of like terms would be [latex]5xy[/latex] and [latex]-3xy[/latex] or [latex]8a^2b[/latex] and [latex]a^2b[/latex] or [latex]-3[/latex] and [latex]8[/latex].  If we have like terms we are allowed to add (or subtract) the numbers in front of the variables, then keep the variables the same. As we combine like terms we need to interpret subtraction signs as part of the following term. This means if we see a subtraction sign, we treat the following term like a negative term. The sign always stays with the term.

This is shown in the following examples:

In the following video you will be shown how to combine like terms using the idea of the distributive property.  Note that this is a different method than is shown in the written examples on this page, but it obtains the same result.

In the video that follows, you will be shown another example of combining like terms.  Pay attention to why you are not able to combine all three terms in the example.

Order of Operations

You may or may not recall the order of operations for applying several mathematical operations to one expression. Just as it is a social convention for us to drive on the right-hand side of the road, the order of operations is a set of conventions used to provide order when you are required to use several mathematical operations for one expression.

In the following example, you will be shown how to simplify an expression that contains both multiplication and subtraction using the order of operations.

When you are applying the order of operations to expressions that contain fractions, decimals, and negative numbers, you will need to recall how to do these computations as well.

In the following video you are shown how to use the order of operations to simplify an expression that contains multiplication, division, and subtraction with terms that contain fractions.

Exponents and Square Roots

In this section, we expand our skills with applying the order of operation rules to expressions with exponents and square roots. If the expression has exponents or square roots, they are to be performed after parentheses and other grouping symbols have been simplified and before any multiplication, division, subtraction, and addition that are outside the parentheses or other grouping symbols.

Recall that an expression such as [latex]7^{2}[/latex] is exponential notation for [latex]7\cdot7[/latex]. (Exponential notation has two parts: the base and the exponent or the power. In [latex]7^{2}[/latex], 7 is the base and 2 is the exponent; the exponent determines how many times the base is multiplied by itself.)

Exponents are a way to represent repeated multiplication; the order of operations places it before any other multiplication, division, subtraction, and addition is performed.

In the video that follows, an expression with exponents on its terms is simplified using the order of operations.

When there are grouping symbols within grouping symbols, calculate from the inside to the outside. That is, begin simplifying within the innermost grouping symbols first.

Remember that parentheses can also be used to show multiplication. In the example that follows, both uses of parentheses—as a way to represent a group, as well as a way to express multiplication—are shown.

In the following video, you are shown how to use the order of operations to simplify an expression with grouping symbols, exponents, multiplication, and addition.

In the next example we will simplify an expression that has a square root.

Some people use a saying to help them remember the order of operations. This saying is called PEMDAS or “Please Excuse My Dear Aunt Sally.” The first letter of each word begins with the same letter of an arithmetic operation.

Please [latex]\displaystyle \Rightarrow[/latex] Parentheses (and other grouping symbols)
Excuse [latex]\displaystyle \Rightarrow[/latex] Exponents
My Dear [latex]\displaystyle \Rightarrow[/latex] Multiplication and Division (from left to right)
Aunt Sally [latex]\displaystyle \Rightarrow[/latex] Addition and Subtraction (from left to right)

Note: Even though multiplication comes before division in the saying, division could be performed first. Which is performed first, between multiplication and division, is determined by which comes first when reading from left to right. The same is true of addition and subtraction. Don’’t let the saying confuse you about this!

Multiple Operations

Next, we will simplify mathematical expressions that contain many grouping symbols and many operations. We will also use the distributive property to break up multiplication into addition.  Additionally, you will see how to handle absolute value terms when you simplify expressions.

The video that follows contains an example similar to the written one above. Note how the numerator and denominator of the fraction are simplified separately.

The Distributive Property

Parentheses are used to group, or combine expressions and terms in mathematics.  You may see them used when you are working with formulas, and when you are translating a real situation into a mathematical problem so you can find a quantitative solution.

Combo Meal Distributive Property

For example, you are on your way to hang out with your friends, and call them to ask if they want something from your favorite drive-through.  Three people want the same combo meal of 2 tacos and one drink.  You can use the distributive property to find out how many total tacos and how many total drinks you should take to them.

[latex]\begin{array}{c}\,\,\,3\left(2\text{ tacos }+ 1 \text{ drink}\right)\\=3\cdot{2}\text{ tacos }+3\text{ drinks }\\\,\,=6\text{ tacos }+3\text{ drinks }\end{array}[/latex]

The distributive property allows us to explicitly describe a total that is a result of a group of groups. In the case of the combo meals, we have three groups of ( two tacos plus one drink). The following definition describes how to use the distributive property in general terms.

To simplify  [latex]3\left(3+y\right)-y+9[/latex], it may help to see the expression translated into words:

multiply three by (the sum of three and y), then subtract y, then add 9

To multiply three by the sum of three and y, you use the distributive property –

[latex]\begin{array}{c}\,\,\,\,\,\,\,\,\,3\left(3+y\right)-y+9\\\,\,\,\,\,\,\,\,\,=\underbrace{3\cdot{3}}+\underbrace{3\cdot{y}}-y+9\\=9+3y-y+9\end{array}[/latex]

Now you can subtract y from 3y and add 9 to 9.

[latex]\begin{array}{c}9+3y-y+9\\=18+2y\end{array}[/latex]

The next example shows how to use the distributive property when one of the terms involved is negative.

Absolute Value

Absolute value expressions are one final method of grouping that you may see. Recall that the absolute value of a quantity is always positive or 0.

When you see an absolute value expression included within a larger expression, treat the absolute value like a grouping symbol and evaluate the expression within the absolute value sign first. Then take the absolute value of that expression. The example below shows how this is done.

The following video uses the order of operations to simplify an expression in fraction form that contains absolute value terms. Note how the absolute values are treated like parentheses and brackets when using the order of operations.

Summary

The order of operations gives us a consistent sequence to use in computation. Without the order of operations, you could come up with different answers to the same computation problem. (Some of the early calculators, and some inexpensive ones, do NOT use the order of operations. In order to use these calculators, the user has to input the numbers in the correct order.)