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7.1 Greatest common factor and factor by grouping

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By the end of this section, you will be able to:
  • Find the greatest common factor of two or more expressions
  • Factor the greatest common factor from a polynomial
  • Factor by grouping

Before you get started, take this readiness quiz.

  1. Factor 56 into primes.
    If you missed this problem, review [link] .
  2. Find the least common multiple of 18 and 24.
    If you missed this problem, review [link] .
  3. Simplify −3 ( 6 a + 11 ) .
    If you missed this problem, review [link] .

Find the greatest common factor of two or more expressions

Earlier we multiplied factors together to get a product. Now, we will be reversing this process; we will start with a product and then break it down into its factors. Splitting a product into factors is called factoring    .

This figure has two factors being multiplied. They are 8 and 7. Beside this equation there are other factors multiplied. They are 2x and (x+3). The product is given as 2x^2 plus 6x. Above the figure is an arrow towards the right with multiply inside. Below the figure is an arrow to the left with factor inside.

We have learned how to factor numbers to find the least common multiple (LCM) of two or more numbers. Now we will factor expressions and find the greatest common factor    of two or more expressions. The method we use is similar to what we used to find the LCM.

Greatest common factor

The greatest common factor    (GCF) of two or more expressions is the largest expression that is a factor of all the expressions.

First we’ll find the GCF of two numbers.

How to find the greatest common factor of two or more expressions

Find the GCF of 54 and 36.

Solution

This table has three columns. In the first column are the steps for factoring. The first row has the first step, factor each coefficient into primes and write all variables with exponents in expanded form. The second column in the first row has “factor 54 and 36”. The third column in the first row has 54 and 36 factored with factor trees. The prime factors of 54 are circled and are 3, 3, 2, and3. The prime factors of 36 are circled and are 2,3,2,3. The second row has the second step of “in each column, circle the common factors. The second column in the second row has the statement “circle the 2, 3 and 3 that are shared by both numbers”. The third column in the second row has the prime factors of 36 and 54 in rows above each other. The common factors of 2, 3, and 3 are circled. The third row has the step “bring down the common factors that all expressions share”. The second column in the third row has “bring down the 2,3, and 3 then multiply”. The third column in the third row has “GCF = 2 times 3 times 3”. The fourth row has the fourth step “multiply the factors”. The second column in the fourth row is blank. The third column in the fourth row has “GCF = 18” and “the GCF of 54 and 36 is 18”.

Notice that, because the GCF is a factor of both numbers, 54 and 36 can be written as multiples of 18.

54 = 18 · 3 36 = 18 · 2
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Find the GCF of 48 and 80.

16

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Find the GCF of 18 and 40.

2

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We summarize the steps we use to find the GCF below.

Find the greatest common factor (gcf) of two expressions.

  1. Factor each coefficient into primes. Write all variables with exponents in expanded form.
  2. List all factors—matching common factors in a column. In each column, circle the common factors.
  3. Bring down the common factors that all expressions share.
  4. Multiply the factors.

In the first example, the GCF was a constant. In the next two examples, we will get variables in the greatest common factor.

Find the greatest common factor of 27 x 3 and 18 x 4 .

Solution

Factor each coefficient into primes and write the variables with exponents in expanded form. Circle the common factors in each column. .
Bring down the common factors. .
Multiply the factors. .
The GCF of 27 x 3 and 18 x 4 is 9 x 3 .

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Find the GCF: 12 x 2 , 18 x 3 .

3 x 2

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Find the GCF: 16 y 2 , 24 y 3 .

8 y 2

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Find the GCF of 4 x 2 y , 6 x y 3 .

Solution

Factor each coefficient into primes and write the variables with exponents in expanded form. Circle the common factors in each column. .
Bring down the common factors. .
Multiply the factors. .
The GCF of 4 x 2 y and 6 x y 3 is 2 xy .

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Find the GCF: 6 a b 4 , 8 a 2 b .

2 a b

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Find the GCF: 9 m 5 n 2 , 12 m 3 n .

3 m 3 n

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Find the GCF of: 21 x 3 , 9 x 2 , 15 x .

Solution

Factor each coefficient into primes and write the variables with exponents in expanded form. Circle the common factors in each column. .
Bring down the common factors. .
Multiply the factors. .
The GCF of 21 x 3 , 9 x 2 and 15 x is 3 x .

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Find the greatest common factor: 25 m 4 , 35 m 3 , 20 m 2 .

5 m 2

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Find the greatest common factor: 14 x 3 , 70 x 2 , 105 x .

7 x

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Factor the greatest common factor from a polynomial

Just like in arithmetic, where it is sometimes useful to represent a number in factored form (for example, 12 as 2 · 6 or 3 · 4 ) , in algebra, it can be useful to represent a polynomial in factored form. One way to do this is by finding the GCF of all the terms. Remember, we multiply a polynomial by a monomial as follows:

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Source:  OpenStax, Elementary algebra. OpenStax CNX. Jan 18, 2017 Download for free at http://cnx.org/content/col12116/1.2
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