Multiply Polynomials — Prealgebra 2e

Multiply a Polynomial by a Monomial

In Distributive Property you learned to use the Distributive Property to simplify expressions such as $2 (x - 3) .$ You multiplied both terms in the parentheses, $x and 3,$ by $2,$ to get $2 x - 6 .$ With this chapter's new vocabulary, you can say you were multiplying a binomial, $x - 3,$ by a monomial, $2 .$ Multiplying a binomial by a monomial is nothing new for you!

Multiply: $3 (x + 7) .$

Solution

	$3 (x + 7)$
Distribute.
	$3 \cdot x + 3 \cdot 7$
Simplify.	$3 x + 21$

Multiply: $x (x - 8) .$

Solution


Distribute.

Simplify.

Multiply: $10 x (4 x + y) .$

Solution


Distribute.

Simplify.

Multiplying a monomial by a trinomial works in much the same way.

Multiply: $−2 x (5 x^{2} + 7 x - 3) .$

Solution

	$−2 x (5 x^{2} + 7 x - 3)$
Distribute.
	$−2 x \cdot 5 x^{2} + (−2 x) \cdot 7 x - (−2 x) \cdot 3$
Simplify.	$−10 x^{3} - 14 x^{2} + 6 x$

Multiply: $4 y^{3} (y^{2} - 8 y + 1) .$

Solution

	$4 y^{3} (y^{2} - 8 y + 1)$
Distribute.
	$4 y^{3} \cdot y^{2} - 4 y^{3} \cdot 8 y + 4 y^{3} \cdot 1$
Simplify.	$4 y^{5} - 32 y^{4} + 4 y^{3}$

Now we will have the monomial as the second factor.

Multiply: $(x + 3) p .$

Solution

	$(x + 3) p$
Distribute.
	$x \cdot p + 3 \cdot p$
Simplify.	$x p + 3 p$

Multiply a Binomial by a Binomial

Just like there are different ways to represent multiplication of numbers, there are several methods that can be used to multiply a binomial times a binomial.

Using the Distributive Property

We will start by using the Distributive Property. Look again at Example 6.


We distributed the $p$ to get
What if we have $(x + 7)$ instead of $p$ ?
Distribute $(x + 7)$ .
Distribute again.	$x^{2} + 7 x + 3 x + 21$
Combine like terms.	$x^{2} + 10 x + 21$

Notice that before combining like terms, we had four terms. We multiplied the two terms of the first binomial by the two terms of the second binomial—four multiplications.

Be careful to distinguish between a sum and a product.

\begin{matrix} Sum & Product \\ x + x & x \cdot x \\ 2 x & x^{2} \\ combine like terms & add exponents of like bases \end{matrix}

Multiply: $(x + 6) (x + 8) .$

Solution

	$(x + 6) (x + 8)$

Distribute $(x + 8)$ .
Distribute again.	$x^{2} + 8 x + 6 x + 48$
Simplify.	$x^{2} + 14 x + 48$

Now we'll see how to multiply binomials where the variable has a coefficient.

Multiply: $(2 x + 9) (3 x + 4) .$

Solution

	$(2 x + 9) (3 x + 4)$
Distribute. $(3 x + 4)$
Distribute again.	$6 x^{2} + 8 x + 27 x + 36$
Simplify.	$6 x^{2} + 35 x + 36$

In the previous examples, the binomials were sums. When there are differences, we pay special attention to make sure the signs of the product are correct.

Multiply: $(4 y + 3) (6 y - 5) .$

Solution

	$(4 y + 3) (6 y - 5)$
Distribute.
Distribute again.	$24 y^{2} - 20 y + 18 y - 15$
Simplify.	$24 y^{2} - 2 y - 15$

Up to this point, the product of two binomials has been a trinomial. This is not always the case.

Multiply: $(x + 2) (x - y) .$

Solution


Distribute.
Distribute again.
Simplify.	There are no like terms to combine.

Using the FOIL Method

Remember that when you multiply a binomial by a binomial you get four terms. Sometimes you can combine like terms to get a trinomial, but sometimes there are no like terms to combine. Let's look at the last example again and pay particular attention to how we got the four terms.

(x + 2) (x - y)

x^{2} - xy + 2 x - 2 y

Where did the first term, $x^{2},$ come from?

It is the product of $x and x,$ the first terms in $(x + 2) and (x - y) .$

Parentheses x plus 2 times parentheses x minus y is shown. There is a red arrow from the first x to the second. Beside this, “First” is written in red.

The next term, $- xy,$ is the product of $x and - y,$ the two outer terms.

Parentheses x plus 2 times parentheses x minus y is shown. There is a black arrow from the first x to the second x. There is a red arrow from the first x to the y. Beside this, “Outer” is written in red.

The third term, $+2 x,$ is the product of $2 and x,$ the two inner terms.

Parentheses x plus 2 times parentheses x minus y is shown. There is a black arrow from the first x to the second x. There is a black arrow from the first x to the y. There is a red arrow from the 2 to the x. Below that, “Inner” is written in red.

And the last term, $−2 y,$ came from multiplying the two last terms.

We abbreviate “First, Outer, Inner, Last” as FOIL. The letters stand for ‘First, Outer, Inner, Last’. The word FOIL is easy to remember and ensures we find all four products. We might say we use the FOIL method to multiply two binomials.

Parentheses a plus b times parentheses c plus d is shown. Above a is first, above b is last, above c is first, above d is last. There is a brace connecting a and d that says outer. There is a brace connecting b and c that says inner.

Let's look at $(x + 3) (x + 7)$ again. Now we will work through an example where we use the FOIL pattern to multiply two binomials.

Comparing the Distributive Property and FOIL method for multiplying binomials, like (x+3)(x+7), both resulting in x^2 + 10x + 21.

Multiply using the FOIL method: $(x + 6) (x + 9) .$

Solution

Step 1: Multiply the First terms.
Step 2: Multiply the Outer terms.
Step 3: Multiply the Inner terms.
Step 4: Multiply the Last terms.
Step 5: Combine like terms, when possible.

We summarize the steps of the FOIL method below. The FOIL method only applies to multiplying binomials, not other polynomials!

Multiply: $(y - 8) (y + 6) .$

Solution

Step 1: Multiply the First terms.
Step 2: Multiply the Outer terms.
Step 3: Multiply the Inner terms.
Step 4: Multiply the Last terms.
Step 5: Combine like terms

Multiply: $(2 a + 3) (3 a - 1) .$

Solution



Multiply the First terms.
Multiply the Outer terms.
Multiply the Inner terms.
Multiply the Last terms.
Combine like terms.

Multiply: $(5 x - y) (2 x - 7) .$

Solution



Multiply the First terms.
Multiply the Outer terms.
Multiply the Inner terms.
Multiply the Last terms.
Combine like terms. There are none.

Using the Vertical Method

The FOIL method is usually the quickest method for multiplying two binomials, but it works only for binomials. You can use the Distributive Property to find the product of any two polynomials. Another method that works for all polynomials is the Vertical Method. It is very much like the method you use to multiply whole numbers. Look carefully at this example of multiplying two-digit numbers.

A vertical multiplication problem is shown. 23 times 46 is written with a line underneath. Beneath the line is 138. Beside 138 is written “partial product.” Beneath 138 is 92. Beside 92 is written “partial product.” Beneath 92 is a line and 1058. Beside 1058 is written “product.”

You start by multiplying $23$ by $6$ to get $138 .$

Then you multiply $23$ by $4,$ lining up the partial product in the correct columns.

Last, you add the partial products.

Now we'll apply this same method to multiply two binomials.

Multiply using the vertical method: $(5 x - 1) (2 x - 7) .$

Solution

It does not matter which binomial goes on the top. Line up the columns when you multiply as we did when we multiplied $23 (46) .$


Multiply $2 x - 7$ by $−1$ .
Multiply $2 x - 7$ by $5 x$ .
Add like terms.

Notice the partial products are the same as the terms in the FOIL method.

We have now used three methods for multiplying binomials. Be sure to practice each method, and try to decide which one you prefer. The three methods are listed here to help you remember them.

Multiply a Trinomial by a Binomial

We have multiplied monomials by monomials, monomials by polynomials, and binomials by binomials. Now we're ready to multiply a trinomial by a binomial. Remember, the FOIL method will not work in this case, but we can use either the Distributive Property or the Vertical Method. We first look at an example using the Distributive Property.

Multiply using the Distributive Property: $(x + 3) (2 x^{2} - 5 x + 8) .$

Solution


Distribute.
Multiply.	$2 x^{3} - 5 x^{2} + 8 x + 6 x^{2} - 15 x + 24$
Combine like terms.	$2 x^{3} + x^{2} - 7 x + 24$

Now let's do this same multiplication using the Vertical Method.

Multiply using the Vertical Method: $(x + 3) (2 x^{2} - 5 x + 8) .$

Solution

It is easier to put the polynomial with fewer terms on the bottom because we get fewer partial products this way.


Multiply $(2 x^{2} - 5 x + 8)$ by 3.
Multiply $(2 x^{2} - 5 x + 8)$ by $x$ .	$A mathematical expression, 2x^3 - 5x^2 + 8x, is displayed above a horizontal line, representing a fraction or a numerator.$
Add like terms.

Key Concepts

Use the FOIL method for multiplying two binomials.

Illustrative steps for applying the FOIL method for multiplying binomials.
Step 1. Multiply the First terms.
Step 2. Multiply the Outer terms.
Step 3. Multiply the Inner terms.
Step 4. Multiply the Last terms.
Step 5. Combine like terms, when possible.

Multiplying Two Binomials: To multiply binomials, use the:
- Distributive Property
- FOIL Method
- Vertical Method
Multiplying a Trinomial by a Binomial: To multiply a trinomial by a binomial, use the:
- Distributive Property
- Vertical Method

Practice Makes Perfect

Multiply a Polynomial by a Monomial

In the following exercises, multiply.

$4 (x + 10)$

Solution

4x + 40

$6 (y + 8)$

$15 (r - 24)$

Solution

15r − 360

$12 (v - 30)$

$−3 (m + 11)$

Solution

−3m − 33

$−4 (p + 15)$

$−8 (z - 5)$

Solution

−8z + 40

$−3 (x - 9)$

$u (u + 5)$

Solution

u² + 5u

$q (q + 7)$

$n (n^{2} - 3 n)$

Solution

n³ − 3n²

$s (s^{2} - 6 s)$

$12 x (x - 10)$

Solution

12x² − 120x

$9 m (m - 11)$

$−9 a (3 a + 5)$

Solution

−27a² − 45a

$−4 p (2 p + 7)$

$6 x (4 x + y)$

Solution

24x² + 6xy

$5 a (9 a + b)$

$5 p (11 p - 5 q)$

Solution

55p² − 25pq

$12 u (3 u - 4 v)$

$3 (v^{2} + 10 v + 25)$

Solution

3v² + 30v + 75

$6 (x^{2} + 8 x + 16)$

$2 n (4 n^{2} - 4 n + 1)$

Solution

8n³ − 8n² + 2n

$3 r (2 r^{2} - 6 r + 2)$

$−8 y (y^{2} + 2 y - 15)$

Solution

−8y³ − 16y² + 120y

$−5 m (m^{2} + 3 m - 18)$

$5 q^{3} (q^{2} - 2 q + 6)$

Solution

5q⁵ − 10q⁴ + 30q³

$9 r^{3} (r^{2} - 3 r + 5)$

$−4 z^{2} (3 z^{2} + 12 z - 1)$

Solution

−12z⁴ − 48z³ + 4z²

$−3 x^{2} (7 x^{2} + 10 x - 1)$

$(2 y - 9) y$

Solution

2y² − 9y

$(8 b - 1) b$

$(w - 6) \cdot 8$

Solution

8w − 48

$(k - 4) \cdot 5$

Multiply a Binomial by a Binomial

In the following exercises, multiply the following binomials using: ⓐ the Distributive Property ⓑ the FOIL method ⓒ the Vertical method

$(x + 4) (x + 6)$

Solution

x² + 10x + 24

$(u + 8) (u + 2)$

$(n + 12) (n - 3)$

Solution

n² + 9n − 36

$(y + 3) (y - 9)$

In the following exercises, multiply the following binomials. Use any method.

$(y + 8) (y + 3)$

Solution

y² + 11y + 24

$(x + 5) (x + 9)$

$(a + 6) (a + 16)$

Solution

a² + 22a + 96

$(q + 8) (q + 12)$

$(u - 5) (u - 9)$

Solution

u² − 14u + 45

$(r - 6) (r - 2)$

$(z - 10) (z - 22)$

Solution

z² − 32z + 220

$(b - 5) (b - 24)$

$(x - 4) (x + 7)$

Solution

x² + 3x − 28

$(s - 3) (s + 8)$

$(v + 12) (v - 5)$

Solution

v² + 7v − 60

$(d + 15) (d - 4)$

$(6 n + 5) (n + 1)$

Solution

6n² + 11n + 5

$(7 y + 1) (y + 3)$

$(2 m - 9) (10 m + 1)$

Solution

20m² − 88m − 9

$(5 r - 4) (12 r + 1)$

$(4 c - 1) (4 c + 1)$

Solution

16c² − 1

$(8 n - 1) (8 n + 1)$

$(3 u - 8) (5 u - 14)$

Solution

15u² − 82u + 112

$(2 q - 5) (7 q - 11)$

$(a + b) (2 a + 3 b)$

Solution

2a² + 5ab + 3b²

$(r + s) (3 r + 2 s)$

$(5 x - y) (x - 4)$

Solution

5x² − 20x − xy + 4y

$(4 z - y) (z - 6)$

Multiply a Trinomial by a Binomial

In the following exercises, multiply using ⓐ the Distributive Property and ⓑ the Vertical Method.

$(u + 4) (u^{2} + 3 u + 2)$

Solution

u³ + 7u² + 14u + 8

$(x + 5) (x^{2} + 8 x + 3)$

$(a + 10) (3 a^{2} + a - 5)$

Solution

3a³ + 31a² + 5a − 50

$(n + 8) (4 n^{2} + n - 7)$

In the following exercises, multiply. Use either method.

$(y - 6) (y^{2} - 10 y + 9)$

Solution

y³ − 16y² + 69y − 54

$(k - 3) (k^{2} - 8 k + 7)$

$(2 x + 1) (x^{2} - 5 x - 6)$

Solution

2x³ − 9x² − 17x − 6

$(5 v + 1) (v^{2} - 6 v - 10)$

Everyday Math

Mental math You can use binomial multiplication to multiply numbers without a calculator. Say you need to multiply $13$ times $15 .$ Think of $13$ as $10 + 3$ and $15$ as $10 + 5 .$

ⓐ Multiply $(10 + 3) (10 + 5)$ by the FOIL method.
ⓑ Multiply $13 \cdot 15$ without using a calculator.

Solution

ⓐ 195
ⓑ 195

Mental math You can use binomial multiplication to multiply numbers without a calculator. Say you need to multiply $18$ times $17 .$ Think of $18$ as $20 - 2$ and $17$ as $20 - 3 .$

ⓐ Multiply $(20 - 2) (20 - 3)$ by the FOIL method.
ⓑ Multiply $18 \cdot 17$ without using a calculator.

Writing Exercises

Which method do you prefer to use when multiplying two binomials—the Distributive Property, the FOIL method, or the Vertical Method? Why?

Solution

Answers will vary.

Which method do you prefer to use when multiplying a trinomial by a binomial—the Distributive Property or the Vertical Method? Why?

Self Check

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

A self-assessment table for students to evaluate their understanding of multiplying polynomials, binomials, and trinomials, with options to rate their skill level as 'Confidently,' 'With some help,' or 'No-I don't get it!'

ⓑ What does this checklist tell you about your mastery of this section? What steps will you take to improve?