Factoring Out the GCF Lesson

In our pre-algebra course, we learned how to find the Greatest Common Factor or GCF for a group of numbers. To find the GCF for a group of numbers, we factor each number completely and then build a list of prime factors that are common to all numbers. The GCF is the product of the numbers in the list.
GCF(12,18,90)
12 = 2 • 2 • 3
18 = 2 • 3 • 3
90 = 2 • 3 • 3 • 5
What's common to everything? Let's organize our factors in a table.

Number	Prime Factors
12	2	2	3
18	2		3	3
90	2		3	3	5

One factor of 2 and one factor of 3 is common to every number.
GCF List: 2, 3
To find the GCF, we multiply the numbers in the GCF list (2,3):
GCF(12,18,90) = 2 • 3 = 6
When we find the GCF for a group of variables, the procedure is the same, however, we can use a shortcut. First and foremost, the variable must appear in each term, secondly, we use the smallest exponent that appears on that variable in any term.
GCF(x⁵, x³, x²y)
We can quickly identify that x is in each term, x⁵, x³, and x²y, however, y is only in one of the terms. The variable x will be in the GCF and the variable y will not be in the GCF. The smallest exponent on x in any of the terms is a 2. This means our GCF will be x².
GCF(x⁵, x³, x²y) = x²
Let's take a look at a few examples.
Example 1: Find the GCF.
GCF(3x²y, 9xyz, 24x⁵y²)
For the number part, our GCF is 3. For the variable part, our GCF is xy. Our GCF is the product of the number part (3) and the variable part (xy).
GCF(3x²y, 9xyz, 24x⁵y²) = 3xy
Example 2: Find the GCF.
GCF(121x⁹yz, 132y³, 209y)
For the number part, our GCF is 11. For the variable part, our GCF is y. Our GCF is the product of the number part (11) and the variable part (y).
GCF(121x⁹yz, 132y³, 209y) = 11y

Factoring out the GCF

When we factor a polynomial, we are writing the polynomial as the product of two or more polynomials. When we factor, we are just reversing the distributive property of multiplication that we learned in pre-algebra.
4(9 + 12) = 4 • 9 + 4 • 12 = 36 + 48
Since we have an equality, we can reverse the process
36 + 48 = 4 • 9 + 4 • 12 = 4(9 + 12)
When we factor out the GCF from a polynomial, we first identify the GCF of all terms of the polynomial. We can then pull this GCF out from each term and place it outside of a set of parentheses. Let's look at a few examples.
Example 3: Factor out the GCF.
65x⁵ - 39x² + 13x
First, we want to find the GCF of all terms.
GCF(65x⁵, 39x², 13x) = 13x
Second, we will rewrite each term as the product of 13x and another factor.
13x • 5x⁴ - 13x • 3x + 13x • 1
Third, we will use our distributive property to factor out the GCF.
13x(5x⁴ - 3x + 1)
Example 4: Factor out the GCF.
95x²y² - 228xy + 133y²
First, we want to find the GCF of all terms.
GCF(95x²y², 228xy, 133y²) = 19y
Second, we will rewrite each term as the product of 19y and another factor.
19y • 5x²y - 19y • 12x + 19y • 7y
Third, we will use our distributive property to factor out the GCF.
19y(5x²y - 12x + 7y)

Factoring out a Common Binomial Factor

In some cases, we will need to factor out a common binomial factor. Let's look at an example.
Example 5: Factor out the GCF.
(x - 7)(x - 12) + (x + 5)(x - 7)
In this case, we can see a common binomial factor of (x - 7). This can be factored in the same way:
(x - 7)(x - 12) + (x + 5)(x - 7)
(x - 7)[(x - 12) + (x + 5)]
We can combine like terms inside of the brackets:
(x - 7)[x - 12 + x + 5]
(x - 7)(2x - 7)

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