Writing Algebraic Expressions
Lesson 2-3
Word phrases can be written as
algebraic expressions.
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Use the words to determine what
operation you are using.
Use a variable for the phrase “a number.”
Adding Phrases
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Plus
More than
Sum
Increased by
Subtracting Phrases
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Minus
Difference
Subtracted from
Decreased by
Less than
Multiplying Phrases
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Times
Product of
Dividing Phrases
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Quotient
Divided by
Example: 7 decreased by b
“Decreased by” means subtract.
7-b
Example: The product of 19 and k
“Product” means multiply. We show multiplication
by placing the factor right next to the variable.
19k
Less than, more than
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Some word phrases contain the words
“less than” or “more than.”
When you see those words, start with the
second part of the phrase.
Example: 23 less than m
m - 23
Example: 32 less than 5 times c
The 32 is less than the 5 times c, which means we
subtract 32 from the 5 times c. So, start with 5 times c.
5c - 32
Tables
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When you are given a table of values,
determine how to get from the first column
to the second column.
Describe that pattern with an algebraic
expression.
Example
n
3
7
11
?
12
28
44
Compare from left to right.
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How do we get from 3 to 12? Add 9 or
multiply by 4.
How do we get from 7 to 28? Multiply by 4.
If we multiply 11 by 4, we get 44.
The pattern is to multiply our number, n, by 4.
The algebraic expression to describe this is:
4n
Describing Unknown Numbers
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When you don’t know what a number is,
let a variable stand for that number.
Use an algebraic expression to describe
what you’re looking for.
Example
Write an algebraic expression to describe Jerry’s age. Use the
following information:
Jerry is 4 years younger than his brother Steve.
First, we have to know how old Steve is. We do not have an
age for Steve, so we will use a variable:
Let s = Steve’s age.
Now that we have determined Steve’s age (s), we can use it
to determine Jerry’s age. Jerry is 4 years younger than Steve.
s-4
If Steve is 22 years old, then how
old is Jerry?
s-4
22 - 4
18
Jerry is 18 years old.
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