1.5.3: Evaluate Expressions with One or More Variables
- Page ID
- 4282
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Dexter is in charge of ticket sales at his town’s water park. He has to report to his boss how many tickets he sells and how much money the water park makes in ticket sales each day - adult tickets ($7), child tickets ($5). Today, Dexter has sold 100 adult and 125 child tickets. Yesterday, he sold 120 adult and 120 child tickets. Can Dexter write an expression to figure out today’s ticket sales, and use this to compare today’s sales to yesterday’s?
In this concept, you will learn how to evaluate expressions that have multiple variables and/or multiple operations.
Expressions with Multiple Variables
When evaluating expressions with multiple variables and multiple operations, it is important to remember the order of operations.
Order of Operations:
P - parentheses
E- exponents
MD - multiplication and division, in order from left to right
AS - addition and subtraction, in order from left to right
Whenever you are evaluating an expression with more than one operation in it, always refer back to the order of operations.
Let's look at an example of an expression with multiple variables and operations.
Evaluate 6a+b when a is 4 and b is 5.
First, you can see that there are two variables in this expression, a and b. There are also two operations here: multiplication, seen in "6 times the value of a" and addition, seen in "+b". You are given values for a and b.
First, substitute the given values for each variable into the expression.
6a+b
6(4)+5
Then, evaluate the expression according to order of operations.
6(4)+5
24+5
29
The answer is 29.
Let’s look at another example with multiple variables and expressions.
Evaluate 7b−d when b is 7 and d is 11.
First, substitute the given values in for the variables.
7(7)−11
Then, evaluate the expression according to order of operations.
49−11
38
The answer is 38.
Sometimes you may have an expression that is all variables. Evaluate this in the same way.
Evaluate ab+cd when a is 4, b is 3, c is 10 and d is 6.
First, substitute the given values in for the variables.
ab+cd
(4)(3)+(10)(6)
Next, evaluate the expression according to order of operations.
12+60
The answer is 72.
Examples
Example \(\PageIndex{1}\)
Earlier, you were given a problem about Dexter and his tickets.
Dexter needs to write an expression to figure out the total money made from the 100 adult and 125 child tickets he sold today, compared to the 120 adult and 120 child tickets he sold yesterday. The adult tickets cost $7 and the child tickets cost $5.
Solution
First, write an expression.
7x+5y
Next, substitute in the given values for today’s ticket sales.
7(100)+5(125)
Then, follow order of operations to multiply and then add.
7(100)+5(125) Multiply 7×100
700+5(125) Multiply 5×125
700+625 Add 700+625=1325
1,325
The answer is 1,325.
Next, substitute in the given values for yesterday’s ticket sales.
7(x)+5(y)
7(120)+5(120)Substitute 120 for both x and y
Then, follow order of operations to multiply and then add.
7(120)+5(120) Multiply 7×120
840+5(120) Multiply 5×120
840+600 Add 840+600=1440
1,440
The answer is 1,440.
Finally, subtract 1,325 from 1,440 to get the difference.
1,440−1,325=115
The answer is 115.
Dexter can report to his boss that today the park sold $1,325 in tickets, which is $115 less than the $1,440 they sold in tickets yesterday.
Example \(\PageIndex{1}\)
Evaluate a+ab+cd when a is 4, b is 9, c is 6 and d is 4.
Solution
First, substitute the given values into the expression.
4+4(9)+6(4)
Next, evaluate according to order of operations.
4+36+24
64
Example \(\PageIndex{1}\)
Evaluate 12x−y when x is 4 and y is 9.
Solution
First, substitute 4 for x and 9 for y.
12x−y Substitute 4 in place of x and 9 for y
12(4)−(9)
Then, evaluate the expression.
12(4)−(9) Multiply 12×4=48
48−(9) Subtract 48−9=39
39
Example \(\PageIndex{1}\)
Evaluate (12/a)+4 when a is 3.
Solution
First, substitute 3 for a.
5x+3y
Next, evaluate the expression.
123+4 Divide 12÷3=4
4+4 Add 4+4=8
8
Example \(\PageIndex{1}\)
Evaluate 5x+3y when x is 4 and y is 8.
Solution
First, substitute 4 for x and 8 for y.
5x+3y Substitute 4 for x and 8 for y
5(4)+3(8)
Then, evaluate the expression.
5(4)+3(8) Multiply 5×4=20
20+3(8) Multiply 3×8=24
20+24 Add 20+24=44
44
The answer is 44.
Review
Evaluate each multi-variable expression when x=2 and y=3.
- 2x+y
- 9x−y
- x+y
- xy
- xy+3
- 9y−5
- 10x−2y
- 3x+6y
- 2x+2y
- 7x−3y
- 3y−2
- 10x−8
- 12x−3y
- 9x+7y
- 11x−7y
Review (Answers)
To see the Review answers, open this PDF file and look for section 1.14.
Vocabulary
| Term | Definition |
|---|---|
| algebraic | The word algebraic indicates that a given expression or equation includes variables. |
| Algebraic Expression | An expression that has numbers, operations and variables, but no equals sign. |
| Exponent | Exponents are used to describe the number of times that a term is multiplied by itself. |
| Expression | An expression is a mathematical phrase containing variables, operations and/or numbers. Expressions do not include comparative operators such as equal signs or inequality symbols. |
| Order of Operations | The order of operations specifies the order in which to perform each of multiple operations in an expression or equation. The order of operations is: P - parentheses, E - exponents, M/D - multiplication and division in order from left to right, A/S - addition and subtraction in order from left to right. |
| Parentheses | Parentheses "(" and ")" are used in algebraic expressions as grouping symbols. |
| revenue | Revenue is money that is earned. |
| substitute | In algebra, to substitute means to replace a variable or term with a specific value. |
| Variable Expression | A variable expression is a mathematical phrase that contains at least one variable or unknown quantity. |
Additional Resources
Videos:
PLIX: Play, Learn, Interact, eXplore: Expressions with One or More Variables: Water Bottle Expression
Activity: Expressions with One or More Variables Discussion Questions
Practice: Evaluate Expressions with One or More Variables
Real World Application: MotoGP