Multiplying Polynomials

Learning Objective(s)

·         Multiply monomials.

·         Multiply monomials times polynomials.

·         Multiply two binomials.

·         Multiply any two polynomials.

Introduction

Multiplying polynomials involves applying the rules of exponents and the distributive property to simplify the product. This multiplication can also be illustrated with an area model, and can be useful in modeling real world situations. Understanding polynomial products is an important step in learning to solve algebraic equations involving polynomials.

Multiplying Monomials

Let's begin by multiplying two simple monomials together. Consider a rectangle whose length is 2x and whose width is 3x. To find the area of this rectangle, multiply the length by the width.

 2x 3x

Area of rectangle = (2x)(3x) = (2x)(3x) = 2 • 3 • xx = 6x2

Note that the commutative and associative properties of multiplication are used to rearrange the factors, putting the coefficients together and the variables together.

The area, 6x2, is a product that includes a coefficient (6) and a variable with a whole number exponent (x2). In other words, it's a monomial, too. So the result of multiplying two monomials is—another monomial!

Let's try a slightly more complex problem: -9x3 • 3x2.

 Example Problem Multiply. -9x3 • 3x2 -9 • 3 • x3 • x2 Use commutative and associative properties of multiplication to rearrange the factors. -27 • x3 • x2 Multiply constants. Remember that a positive number times a negative number yields a negative number. -27 • x3+2 -27 • x5 Multiply variable terms. Remember to add the exponents when multiplying exponents with the same base. Answer −9x3 • 3x2 = −27x5

That’s it! When multiplying monomials, multiply the coefficients together, and then multiply the variables together. If two variables have the same base, follow the rules of exponents, like this:

 Find the area of the rectangle:       A) 8y3   B) 15y5   C) 15y10   D) 8y5   Show/Hide Answer

The Product of a Monomial and a Polynomial

The distributive property can be used to multiply a polynomial by a monomial. Just remember that the monomial must be multiplied by each term in the polynomial. Consider the expression 2x(2x2 + 5x + 10).

This expression can be modeled with a sketch like the one below.

 2x2 5x 10 2x 4x3 10x2 20x

The model above illustrates the distributive property.

2x(2x2 + 5x + 10) = 2x(2x2) + 2x(5x) = 2x(10)

=       4x3 + 10x2 + 20x

Here’s an example:

 Example Problem Simplify. 5x(4x2 + 3x + 7) 5x(4x2) + 5x(3x) + 5x(7) Distribute the monomial to each term of the polynomial. 20x3 + 15x2 + 35x Multiply. Answer 5x(4x2 + 3x + 7) = 20x3 + 15x2 + 35x

You may need to rewrite subtraction as adding the opposite.

 Example Problem Simplify. 7x2(2x2 – 5x + 1) 7x2[2x2 + (– 5x) + 1] Rewrite the subtraction as adding the opposite. 7x2(2x2) +  7x2(– 5x) + 7x2(1) Distribute the monomial to each term of the polynomial. 14x4 + (-35)x3 + 7x2 Multiply. Answer 7x2(2x2 – 5x + 1) = 14x4 – 35x3 + 7x2 Rewrite addition of terms with negative coefficients as subtraction.

 Find the product. Watch the signs! -3t2(7t3 + 3t2 – t)   A) -21t5 – 9t4 + 3t3   B) -21t5 + 9t4 – 3t3   C) -21t6 – 9t4 + 3t2   D) -21t5 + 3t2 – t   Show/Hide Answer

Product of Two Binomials

Now let's explore multiplying two binomials. Once again, you can draw an area model to help make sense of the process. You'll use each binomial as one of the dimensions of a rectangle, and their product as the area.

The model below shows (x + 4)(2x + 2):

 x + 4 2x 2x2 8x + 2 2x 8

Each binomial is expanded into variable terms and constants, x + 4, along the top of the model and 2x + 2 along the left side. The product of each pair of terms is a colored rectangle. The total area is the sum of all of these small rectangles, 2x2 + 8x + 2x + 8, If you combine all the like terms, you can write the product, or area, as 2x2 + 10x + 8.

You can use the distributive property to determine the product of two binomials.

 Example Problem (x + 4)(2x + 2) x(2x) + x(2) +  4(2x) + 4(2) Distribute the x over 2x + 2, then distribute 4 over 2x + 2. 2x2 + 2x + 8x + 8 Multiply. 2x2 + 10x + 8 Combine like terms (8x + 2x). Answer (x + 4)(2x + 2) = 2x2 + 10x + 8

Look back at the model above to see where each piece of 2x2 + 8x + 2x + 8 comes from. Can you see where you multiply x by 2x + 2, and where you get 2x2 from x(2x)?

Another way to look at multiplying binomials is to see that each term in one binomial is multiplied by each term in the other binomial. Look at the example above: the x in x + 4 gets multiplied by both the 2x and the 2 from 2x + 2, and the 4 gets multiplied by both the 2x and the 2.

Some people use the FOIL method to keep track of which pairs have been multiplied. The letters in FOIL stand for First, Outer, Inner, Last:

First term in each binomial:    (x + 4)(2x + 2)             x(2x) = 2x2

Outer terms:                           (x + 4)(2x + 2)             x(2) = 2x

Inner terms:                            (x + 4)(2x + 2)             4(2x) = 8x

Last terms in each binomial:  (x + 4)(2x + 2)             4(2) = 8

2x2 + 2x + 8x + 8 = 2x2 + 10x + 8.

Here is another example – this time using FOIL.

 Example Problem (4x – 10)(2x + 3) 4x(2x) = 8x2 4x(3) = 12x −10(2x) = -20x −10(3) = -30 First Outer Inner Last   Be careful about including the negative sign on the ‑10, since 10 is subtracted. 8x2 + 12x – 20x – 30 Combine like terms. Answer (4x – 10)(2x + 3) = 8x2 – 8x – 30

Because multiplication is commutative, the terms can be multiplied in either order. The expression (2x + 2)(x + 4) has the same product as (x  + 4)(2x + 2), 2x2 + 10x + 8. (Work it out and see.) The order in which you multiply binomials does not matter. What matters is that you multiply each term in one binomial by each term in the other binomial.

The last step in multiplying polynomials is to combine like terms. Remember that a polynomial is simplified only when there are no like terms remaining.

 Find the product: (a + 10)(2a – 7)   A) 2a2 + 19a – 70   B) 3a + 3   C) 2a2 – 70   D) 2a2 + 13a – 70   Show/Hide Answer

Product of a Binomial and a Trinomial

Another type of polynomial multiplication problem is the product of a binomial and trinomial. Although the FOIL method can not be used exactly since there are more than two terms in a trinomial, you still use the Distributive Property to organize the individual products. Using the distributive property, each term in the binomial must be multiplied by each of the terms in the trinomial. Two examples are shown below.

 Example Problem (3x + 6)(5x2 + 3x + 10) 3x(5x2 + 3x + 10) + 6(5x2 + 3x + 10) Distribute the trinomial to each term in the binomial. 3x(5x2) + 3x(3x) + 3x(10) + 6(5x2) + 6(3x) + 6(10) Use the distributive property to distribute the monomials to each term in the trinomials. 15x3 + 9x2 + 30x + 30x2 + 18x + 60 Multiply. 15x3 + (9x2 + 30x2) + (30x + 18x) + 60 Group like terms. Answer (3x + 6)(5x2 + 3x + 10) = 15x3 + 39x2 + 48x + 60 Combine like terms.

As you can see, multiplying a binomial by a trinomial leads to a lot of individual terms! Some people prefer to set up these problems vertically and gather like terms as they multiply. This method is shown below, using the same problem as above.

Example

Problem

(3x + 6)(5x2 + 3x + 10)

 3x + 6 x 5x2 + 3x + 10 + 30x +  60

Set up the problem in a vertical form, and begin by multiplying 3x + 6 by + 10. Place the products underneath, as shown.

 3x + 6 x 5x2 + 3x + 10 + 30x + 60 + 9x2 + 18x

Now multiply 3x + 6 by + 3x. Notice that (6)(3x) = 18x; since this term is like 30x, place it directly beneath it.

 3x + 6 x 5x2 + 3x + 10 + 30x + 60 + 9x2 + 18x + 15x3 + 30x2

Finally, multiply 3x + 6 by 5x2. Notice that 30x2 is placed underneath 9x2.

 3x + 6 x 5x2 + 3x + 10 + 30x +  60 + 9x2 + 18x + 15x3 + 30x2 + 15x3 + 39x2 + 48x + 60

15x3 + 39x2 + 48x + 60

Notice that although the two problems were solved using different strategies, the product is the same. Both the horizontal and vertical methods apply the Distributive Property to multiply a binomial by a trinomial.

The next example shows multiplication by a binomial and trinomial that each contains subtraction. The example completes the multiplication without rewriting each subtraction as addition of the opposite. Notice where you must be careful with the signs! (If you prefer, you can continue to rewrite subtraction as adding the opposite.)

 Example Problem (2p – 1)(3p2 – 3p ­+ 1) 2p(3p2 – 3p + 1) – 1(3p2 – 3p + 1) Distribute the trinomial to each term in the binomial. 2p(3p2) + 2p(-3p) + 2p(1) – 1(3p2) – 1(-3p) – 1(1) If you don’t write subtraction as adding the opposite, then be sure to think of it that way. So you are distributing -1 and multiplying each term of the trinomial by -1. 6p3 – 6p2 + 2p – 3p2 + 3p – 1 Multiply. (Notice that the subtracted 1 and the subtracted 3p have a positive product that is added.) 6p3 – 9p2 + 5p – 1 Combine like terms. Answer 6p3 – 9p2 + 5p – 1

 Find the product: (3x – 2)(2x2 + 4x – 11)   A) 6x3 + 8x2 – 41x + 22   B) 6x3 + 8x2 – 41x – 22   C) 6x3 + 12x + 22   D) 3x3 + 8x2 + 25x – 22   Show/Hide Answer

Summary

Multiplication of binomials and polynomials requires use of the distributive property as well as the commutative and associative properties of multiplication. Whether the polynomials are monomials, binomials, or trinomials, carefully multiply each term in one polynomial by each term in the other polynomial. Be careful to watch the addition and subtraction signs and negative coefficients. A product is written in simplified form if all of its like terms have been combined.