### Learning Objectives

- Find the greatest common factor of two or more expressions
- Factor the greatest common factor from a polynomial

Before you get started, take this readiness quiz.

- Factor $56$ into primes.

If you missed this problem, review Example 2.48. - Multiply: $\mathrm{-3}\left(6a+11\right).$

If you missed this problem, review Example 7.25. - Multiply: $4{x}^{2}\left({x}^{2}+3x-1\right).$

If you missed this problem, review Example 10.32.

### 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.

In The Language of Algebra we factored 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 will find the greatest common factor of two numbers.

### Example 10.80

Find the greatest common factor of $24$ and $36.$

Find the greatest common factor: $54,36.$

Find the greatest common factor: $48,80.$

In the previous example, we found the greatest common factor of constants. The greatest common factor of an algebraic expression can contain variables raised to powers along with coefficients. We summarize the steps we use to find the greatest common factor.

### How To

#### Find the greatest common factor.

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

### Example 10.81

Find the greatest common factor of $5x\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}15.$

Find the greatest common factor: $7y,\phantom{\rule{0.2em}{0ex}}14.$

Find the greatest common factor: $22,\phantom{\rule{0.2em}{0ex}}11m.$

In the examples so far, the greatest common factor was a constant. In the next two examples we will get variables in the greatest common factor.

### Example 10.82

Find the greatest common factor of $12{x}^{2}$ and $18{x}^{3}.$

Find the greatest common factor: $16{x}^{2},\phantom{\rule{0.2em}{0ex}}24{x}^{3}.$

Find the greatest common factor: $27{y}^{3},\phantom{\rule{0.2em}{0ex}}18{y}^{4}.$

### Example 10.83

Find the greatest common factor of $14{x}^{3},\phantom{\rule{0.2em}{0ex}}8{x}^{2},\phantom{\rule{0.2em}{0ex}}10x.$

Find the greatest common factor: $21{x}^{3},\phantom{\rule{0.2em}{0ex}}9{x}^{2},\phantom{\rule{0.2em}{0ex}}15x.$

Find the greatest common factor: $25{m}^{4},\phantom{\rule{0.2em}{0ex}}35{m}^{3},\phantom{\rule{0.2em}{0ex}}20{m}^{2}.$

### 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 $2\xb76\phantom{\rule{0.2em}{0ex}}\text{or}\phantom{\rule{0.2em}{0ex}}3\xb74\text{),}$ in algebra it can be useful to represent a polynomial in factored form. One way to do this is by finding the greatest common factor of all the terms. Remember that you can multiply a polynomial by a monomial as follows:

Here, we will start with a product, like $2x+14,$ and end with its factors, $2\left(x+7\right).$ To do this we apply the Distributive Property “in reverse”.

### Distributive Property

If $a,\phantom{\rule{0.2em}{0ex}}b,\phantom{\rule{0.2em}{0ex}}c$ are real numbers, then

The form on the left is used to multiply. The form on the right is used to factor.

So how do we use the Distributive Property to factor a polynomial? We find the GCF of all the terms and write the polynomial as a product!

### Example 10.84

Factor: $2x+14.$

Factor: $4x+12.$

Factor: $6a+24.$

Notice that in Example 10.84, we used the word *factor* as both a noun and a verb:

### How To

#### Factor the greatest common factor from a polynomial.

- Step 1. Find the GCF of all the terms of the polynomial.
- Step 2. Rewrite each term as a product using the GCF.
- Step 3. Use the Distributive Property ‘in reverse’ to factor the expression.
- Step 4. Check by multiplying the factors.

### Example 10.85

Factor: $3a+3.$

Factor: $9a+9.$

Factor: $11x+11.$

The expressions in the next example have several factors in common. Remember to write the GCF as the product of all the common factors.

### Example 10.86

Factor: $12x-60.$

Factor: $11x-44.$

Factor: $13y-52.$

Now we’ll factor the greatest common factor from a trinomial. We start by finding the GCF of all three terms.

### Example 10.87

Factor: $3{y}^{2}+6y+9.$

Factor: $4{y}^{2}+8y+12.$

Factor: $6{x}^{2}+42x-12.$

In the next example, we factor a variable from a binomial.

### Example 10.88

Factor: $6{x}^{2}+5x.$

Factor: $9{x}^{2}+7x.$

Factor: $5{a}^{2}-12a.$

When there are several common factors, as we’ll see in the next two examples, good organization and neat work helps!

### Example 10.89

Factor: $4{x}^{3}-20{x}^{2}.$

Factor: $2{x}^{3}+12{x}^{2}.$

Factor: $6{y}^{3}-15{y}^{2}.$

### Example 10.90

Factor: $21{y}^{2}+35y.$

Factor: $18{y}^{2}+63y.$

Factor: $32{k}^{2}+56k.$

### Example 10.91

Factor: $14{x}^{3}+8{x}^{2}-10x.$

Factor: $18{y}^{3}-6{y}^{2}-24y.$

Factor: $16{x}^{3}+8{x}^{2}-12x.$

When the leading coefficient, the coefficient of the first term, is negative, we factor the negative out as part of the GCF.

### Example 10.92

Factor: $\mathrm{-9}y-27.$

Factor: $\mathrm{-5}y-35.$

Factor: $\mathrm{-16}z-56.$

Pay close attention to the signs of the terms in the next example.

### Example 10.93

Factor: $\mathrm{-4}{a}^{2}+16a.$

Factor: $\mathrm{-7}{a}^{2}+21a.$

Factor: $\mathrm{-6}{x}^{2}+x.$

### Media Access Additional Online Resources

### Section 10.6 Exercises

#### Practice Makes Perfect

**Find the Greatest Common Factor of Two or More Expressions**

In the following exercises, find the greatest common factor.

$40,56$

$72,162$

$3x,12$

$10a,50$

$16y,\phantom{\rule{0.2em}{0ex}}24{y}^{2}$

$18{m}^{3},\phantom{\rule{0.2em}{0ex}}36{m}^{2}$

$10x,\phantom{\rule{0.2em}{0ex}}25{x}^{2},\phantom{\rule{0.2em}{0ex}}15{x}^{3}$

$24u,\phantom{\rule{0.2em}{0ex}}6{u}^{2},\phantom{\rule{0.2em}{0ex}}30{u}^{3}$

$15{a}^{4},\phantom{\rule{0.2em}{0ex}}9{a}^{5},\phantom{\rule{0.2em}{0ex}}21{a}^{6}$

$27{y}^{2},\phantom{\rule{0.2em}{0ex}}45{y}^{3},\phantom{\rule{0.2em}{0ex}}9{y}^{4}$

**Factor the Greatest Common Factor from a Polynomial**

In the following exercises, factor the greatest common factor from each polynomial.

$2x+8$

$3a-24$

$9y-9$

$5{m}^{2}+20m+35$

$8{p}^{2}+32p+48$

$8{q}^{2}+15q$

$13{k}^{2}+5k$

$5{c}^{2}+9c$

$5{p}^{2}+25p$

$24{q}^{2}-12q$

$yz+4z$

$60x-6{x}^{3}$

$48{r}^{4}-12{r}^{3}$

$4{a}^{3}-4a{b}^{2}$

$30{u}^{3}+80{u}^{2}$

$120{y}^{6}+48{y}^{4}$

$4{q}^{2}+24q+28$

$15{z}^{2}-30z-90$

$3{a}^{4}-24{a}^{3}+18{a}^{2}$

$11{x}^{6}+44{x}^{5}-121{x}^{4}$

$\mathrm{-3}n-24$

$\mathrm{-15}{a}^{2}-40a$

$\mathrm{-10}{y}^{3}+60{y}^{2}$

$\mathrm{-4}{u}^{5}+56{u}^{3}$

#### Everyday Math

**Revenue** A manufacturer of microwave ovens has found that the revenue received from selling microwaves a cost of $p$ dollars each is given by the polynomial $\mathrm{-5}{p}^{2}+150p.$ Factor the greatest common factor from this polynomial.

**Height of a baseball** The height of a baseball hit with velocity $80$ feet/second at $4$ feet above ground level is $\mathrm{-16}{t}^{2}+80t+4,$ with $t=$ the number of seconds since it was hit. Factor the greatest common factor from this polynomial.

#### Writing Exercises

The greatest common factor of $36$ and $60$ is $12.$ Explain what this means.

What is the GCF of ${y}^{4}$, ${y}^{5}$, and ${y}^{10}$? Write a general rule that tells how to find the GCF of ${y}^{\text{a}}$, ${y}^{\text{b}}$, and ${y}^{\text{c}}$.

#### Self Check

ⓐ After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.

ⓑ Overall, after looking at the checklist, do you think you are well-prepared for the next Chapter? Why or why not?