Main content

### Course: Algebra 1 > Unit 1

Lesson 2: Introduction to variables- What is a variable?
- Why aren't we using the multiplication sign?
- Creativity break: Why is creativity important in STEM jobs?
- Evaluating an expression with one variable
- Evaluating expressions with one variable
- Evaluating expressions with one variable

© 2024 Khan AcademyTerms of usePrivacy PolicyCookie Notice

# Evaluating expressions with one variable

A mixture of explanations, examples, and practice problems to have you evaluating expressions with one variable in no time!

## How to evaluate an expression with one variable

Let's say we want to evaluate the expression $a+4$ . Well, first we need to know the value of the variable $a$ . For example, to evaluate the expression when ${a=1}$ , we just replace ${a}$ with ${1}$ :

So, the expression $a+4$ equals $5$ when $a=1$ .

We can just as easily evaluate $a+4$ when ${a=5}$ :

So, the expression $a+4$ equals $9$ when $a=5$ .

## Evaluating an expression with multiplication

You might be asked to "

**Evaluate**$3x$ when $x=5$ ."Notice how the number $3$ is right next to the variable $x$ in the expression $3x$ . This means "$3$ times $x$ ". The reason we do this is because the old way of showing multiplication with the symbol $\times $ looks confusingly similar to the variable $x$ .

Okay, so now let's solve the problem:

So, the expression $3x$ equals $15$ when $x=5$ .

### New ways to show multiplication

Hold on a second! Did you notice that we wrote "$3$ times ${5}$ " as $3\cdot {5}$ instead of as $3\times {5}$ ? Using a dot instead of the symbol $\times $ is another new way of showing multiplication:

Parentheses can also be used to show multiplication:

Let's summarize the new ways of showing multiplication that we learned.

Old way | New way | |
---|---|---|

With a variable | ||

Without variable |

## Evaluating expressions where order of operations matter

For more complex expressions, we'll have to be sure to pay close attention to order of operations. Let's take a look at an example:

**Evaluate**$5+3e$ when ${e=4}$ .

So, the expression $5+3e$ equals $17$ when $e=4$ .

Notice how we had to be careful to think about order of operations when evaluating. A common wrong answer is ${32}$ , which comes from first adding $5$ and $3$ to get $8$ then multiplying $8$ by $4$ to get ${32}$ .

## Let's practice!

## Challenge problems

## Want to join the conversation?

- I still don't understand the challenge problems e⋅e−5ee, dot, e, minus, 5, e when e=5e=5e, equals, 5.What does this mean?(62 votes)
- so, e(dot)e - 5e, the dot symbol represents multiplication. And 5e is another way to represent multiplication so, the expression actually looks like this: 5x5 - 5x5 which equals to 0 because 5x5 = 25, and 5x5 = 25, and subtracting 25 with 25 = 0.

e(dot)e - 5e

5x5 - 5x5 = 25 - 25

25 - 25 = 0(137 votes)

- If 9-8/x = ? and x=4 what do I do with the 8/x? Do I multiply add divide or subtract?(35 votes)
- you substitute for x, so 9 - 8/4, do divide (8/4) first then subtract from 9.(51 votes)

- Why do we have to do multiplication and division first? Why to use these "order of operation"?(9 votes)
- Here is why; "Because exponents are simply repeated multiplication, they are performed before multiplication. ... Thus the order: parentheses first, exponents second, mutiplication and division third, and save the lowest level operations of addition and subtraction for last." Personally I believe order of operation is way to balance the equation as well. :)(36 votes)

- If you don't know what y is, then you won't get an exact answer(18 votes)
- Well you don't need to know what y is to solve the question. For example you have:6t=48. (if something is written like 6t, that always means you have to multiply, or any variable). So we have: 6t=48, we can divide both sides by 6 (we are dividing because you always do the inverse of what you are trying to solve, the opposite of multiplication is division)

we have to divide both sides by 6

6t=48, 6/6 cancels out, 48/6=8. So now the equation would look like this: t=8, and now we know what t is we can verify it. 6t=48 or 6x8=48.

And that's the beauty of Algebra!

If you need more help just tell me.

-Intellectual Genius(32 votes)

- what is it asking me when it says Evaluate 2y2y when y = 6y=6(15 votes)
- It means you use substitution... You replace the "y" in "2y" with the value you are given. Since you were told to use "y=6", you take out "y" and replace it with 6. Remember, "2y" means "2 times y", so you need a multiplication symbol. Thus, "2y" becomes "2(6)".

Then, you simplify the expression by doing the multiplication. Your final answer would be 12.

Hope this helps.(23 votes)

- i can't understand the question nine please help me understand(17 votes)
- Start out with the variable. What is d worth? What do you need to do with that variable in the sentence? You should see that d is worth 4, and we need to now add that in the sentence.

The sentence should now look like this -

8/4 + 3

Now, solve it like you would usually do it. 8/4 is equal to 2 (We need to do it first since it is the second in the Order of Operations), and then add 3 to that 2. You should get an answer of 5 if you did all that calculation correctly.

Hope this helped!(14 votes)

- How do you do this with fractions? (I'm really bad with fractions and math)(11 votes)
- The process is no different. You replace each variable with its given value. Then you simplify the expression following order of operations rules and the rules for working with fractions. If you aren't great at working with fractions, then do yourself a favor and get more practice on that topic.(11 votes)

- is it just me or is it way easier here than in class?(18 votes)
- waaaaaaaaaaaaaay more easier than class. I think they make it dificult on purpouse so they look smarter!(5 votes)

- Does this topic have viable application to non-Algebra subjects, if any? Thanx.(11 votes)
- Absolutely! There are many formulas that get used in real life situations. This teaches you how to use a formula and calculate a result based upon replacing values into the formula.(20 votes)

- so u saying if i put 5 + 2b and be equal to 5 the answer would be 15(14 votes)
- If b=5 then yes it does. it means this 5+2x5(6 votes)