Expressing Relationships as Algebraic Expressions

Expressing Relationships as Algebraic Expressions

What do you do when you don't know what a number is but you do know how it relates to something else? You use an algebraic expression. In this lesson, we'll learn how to express relationships as algebraic expressions.

Expressions and Variables

o you ever have trouble putting what you want to say into words? Maybe you're trying to explain something and it's like you're speaking in another language that the other person can't understand. I think this is how my high school chemistry teacher felt while teaching me. Or maybe you're trying to tell someone you, you know, like them, or like like them, but he or she just isn't hearing you. It's frustrating, I know. Or, I mean, you know, I've heard.

Life would sometimes be simpler if we could just use math to speak for us all the time. Fortunately, there are endless real-life situations we can express using algebra. We just need a few tools. First, of course, are numbers. This is math, after all. We also need variables. A variable is a symbol that represents an unknown number. Then we'll need some operators, like addition and division. With these tools, we can say all kinds of things with ease.

Algebraic Expressions

We use the tools to build algebraic expressions. An algebraic expression is a mathematical phrase that may include numbers, variables and operators. It's basically like a sentence. But you're substituting these numbers, variables and operators for words.

Algebraic expressions can look like x + 1, 17y, 4a - 3 or q/6.

Addition and Subtraction

Let's start with addition and subtraction expressions. Here's one: There are two competing lemonade stands run by siblings April and Mike. We want to describe the relationship between the prices for a cup of lemonade between the two stands. April is selling her lemonade for 50 cents less than Mike's. What do we do?

First, we need a variable for the cost of April's lemonade. Let's call that a. We use that in place of a number we don't know. Now, if Mike's lemonade is 50 cents more than April's, we can describe his as a + 50. That a+ 50 describes the cost of Mike's lemonade relative to the cost of April's.

We can test our expression by substituting a number for our variable. Let's say a = 75 cents. a + 50 = $1.25. Is $1.25 50 cents more than 75 cents? Yes! So we know that we have the correct expression.

We could also use subtraction here. We could use m as the cost of Mike's lemonade. Since April's is 50 cents less than Mike's, hers would be m - 50. Again, we could test this. Let's say m = 80 cents. So m - 50 = 30 cents. Is 30 50 cents less than 80? It is. It's also a very cheap cup of lemonade.

Multiplication and Division

Next, let's look at multiplication. Let's say it's the holiday season and you're buying gifts for your siblings. Since you live in an algebra problem, let's say you have 15 siblings. That's a lot, so you have to keep these gifts small.

Let's say you can spend d dollars on each gift. How do you describe the total cost of the gifts? We could add them together and say d + d + d +... well, 15 total ds. It would be simpler to just say 15 times d, or 15d. That's the cost of one gift times 15. So if you spent $10 per gift, you'd spend 15 times 10, which is $150.

Here's one that involves division. Let's say you're working with a study group. You all get hungry and order some pizza. There are three of you. If everyone's going to get the same amount, how much pizza can you eat? Well, you don't know how many slices there will be. So let's use s to stand in for the total number of slices. Since there are 3 of you, each of you gets s/3 slices. That's the total number of slices divided by 3. If there were 9 slices, you'd get 9 divided by 3, or 3 slices each.

Multiple Variables

So far, all of our expressions used just one variable. But life isn't always that simple. Let's say you're going on a date. It's dinner and a movie. You want to express how much this date will cost.

Entrees cost e and movie tickets cost t. How do you write this? Well, there are two of you, so two entrees will be 2e. And supposing your date doesn't bail on you before the movie, you'll need two tickets or 2t. The total cost is 2e + 2t.

Sometimes we need to mix operators as well as variables. Don't worry. You can handle this. Let's say you don't have one date this week, but three. How do you show that? Just take our expression, 2e + 2t, and multiply the whole thing by 3 - so 3(2e + 2t).

Lesson Summary

In summary, an algebraic expression is a mathematical phrase that may include numbers, variables, and operators. We use variables in place of the numbers we don't know. Then, using our operators, we explain the relationships between different values.