Lesson Explainer: Simplifying Monomials: Multiplication | Nagwa Lesson Explainer: Simplifying Monomials: Multiplication | Nagwa

Lesson Explainer: Simplifying Monomials: Multiplication Mathematics

In this explainer, we will learn how to multiply monomials involving single and multiple variables.

Algebraic expressions allow us to represent unknown values. This means that manipulating and simplifying algebraic expressions can help us rewrite or simplify these unknown values expressed in terms of some variables.

In this explainer, we will focus on simplifying the product of monomial terms. To do this, let’s start by recalling the definition of a monomial.

Definition: Monomial

A monomial is a single algebraic term where every variable is raised to a nonnegative integer power.

For example, 5𝑥 is a monomial since it is a single term and the variable 𝑥 is raised to a nonnegative integer power; 𝑥=𝑥. Similarly, 12𝑥𝑦 is a monomial since it is a single term and every variable is raised to a nonnegative integer power.

We want to multiply two monomials. We can do this by considering an example. Let’s say we want to multiply 5𝑥 by 3𝑥. We have 5𝑥×3𝑥=(5×𝑥)×(3×𝑥).

Remember that 𝑥 represents a number, so we can use here the commutative and associative properties of multiplication to rewrite this product as follows: (5×𝑥)×(3×𝑥)=(5×3)×(𝑥×𝑥).

The product of the coefficients is 5×3=15. Similarly, we can remember that the repeated multiplication 𝑥×𝑥 can be written as 𝑥.

Hence, we have shown that 5𝑥×3𝑥=15𝑥.

We evaluated this product of monomials by multiplying the coefficients and the factors of 𝑥 separately.

When we multiplied the factors of 𝑥 in this expression, we saw that 5𝑥 had 2 factors of 𝑥 and 3𝑥 has 1 factor of 𝑥. We can add the number of factors together to get the number of factors of 𝑥 in the product:

We can note that this process will work in general, since we add the number of factors to determine the power. This is called the product rule for powers, and we can prove this result following the same reasoning.

Let 𝑏 and 𝑚 and 𝑛 be nonnegative integers. We can multiply 𝑏 by 𝑏 by expanding the powers and then adding the number of factors of 𝑏: 𝑏𝑏=(𝑏𝑏𝑏𝑏𝑏)(𝑏𝑏𝑏𝑏)=𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏=𝑏.factorsoffactorsoffactorsof

We have shown the following result.

Rule: Product Rule for Exponents

For any rational number 𝑏 and nonnegative integers 𝑚 and 𝑛, we have 𝑏×𝑏=𝑏.

It is important to understand here why the product rule is true only if the two powers have the same base. Let’s look at an example: 2×5. We can write 2×5=2×2×2×5×5, and here, we see that we are stuck; we cannot simplify this expression because the bases are different (2 and 5).

This holds true for any base𝑏. This means it will also hold true if we have a variable as a base.

In our first example, we will apply the product rule to simplify the product of two monomials.

Example 1: Applying the Product Rule

Simplify 𝑏×𝑏.

Answer

We want to simplify this product of two powers. We see that the base (𝑏) is the same in both factors and both powers are nonnegative integers. Therefore, we can apply the product rule, which tells us that for any rational number 𝑏 and nonnegative integers 𝑚 and 𝑛, we have 𝑏×𝑏=𝑏.

In other words, the product of two powers that have the same base can be expressed as a power of this base with an exponent equal to the sum of the exponents.

Hence, we add the exponents to get 𝑏×𝑏=𝑏=𝑏.

In our next example, we will find the product of two monomials with variables raised to powers greater than 1.

Example 2: Multiplying Monomials Using Laws of Exponents

Simplify 7𝑥×8𝑥.

Answer

In this question, we want to simplify the product of two monomials. To make the product clearer, we can rewrite this product by including the multiplication signs that are implicit within the monomials, that is, between the coefficient and the variables: 7𝑥×8𝑥=7×𝑥×8×𝑥.

Now, we can use the commutativity and associativity of multiplication to rearrange our right-hand side expression: 7×𝑥×8×𝑥=7×8×𝑥×𝑥.

Now, we multiply the coefficients together: 7×8×𝑥×𝑥=56×𝑥×𝑥.

Since the bases in the final two factors are both 𝑥 and the powers are nonnegative integers, the product rule for exponents tells us that we can evaluate their product by adding the exponents. We have 56×𝑥×𝑥=56×𝑥=56𝑥.

The product of monomials is not limited to positive integer coefficients, as we will see in our next example.

Example 3: Multiplying Monomials with Fractional Coefficients

Simplify 13𝑥×45𝑥.

Answer

We first rewrite the product, using the commutativity and associativity of multiplication, as 13𝑥×45𝑥=13×45×𝑥×𝑥.

Next, we can evaluate each product inside the parentheses separately. To multiply two fractions, we multiply the numerators and denominators. We have 13×45=1×43×5=415.

To evaluate 𝑥×𝑥, we need to use the product rule for exponents, which tells us that we can evaluate this product by adding the exponents. This gives 𝑥×𝑥=𝑥=𝑥.

Substituting these back into the expression gives 13𝑥×45𝑥=415𝑥.

In our next example, we will use this process for multiplying monomials to simplify an expression for the volume of a given rectangular prism.

Example 4: Writing an Algebraic Expression for the Volume of a Rectangular Prism by Multiplying Monomials

Find an expression for the volume of the rectangular prism shown.

Answer

We first recall that the volume of a rectangular prism is given by the product of its length, width, and height. If we call this volume 𝑉, we have 𝑉=15𝑥×10𝑥×3𝑥.

We can simplify this expression by noting that this is the product of three monomials. This means we can multiply the coefficients and variables separately by using the commutativity and associativity of multiplication. We have 15𝑥×10𝑥×3𝑥=(15×10×3)×(𝑥×𝑥×𝑥).

We then note that the repeated multiplication 𝑥×𝑥×𝑥 can be written as 𝑥. Thus, 𝑉=450𝑥.

We can use the product rule for exponents to simplify the product of any monomials even if they have multiple variables. For example, let’s say we want to find the product of 3𝑥𝑦 and 12𝑥𝑦. We can start by using commutativity and associativity of multiplication to rewrite the product as 3𝑥𝑦×12𝑥𝑦=3×12×𝑥×𝑥×𝑦×𝑦.

Now, we can evaluate each product inside the parentheses separately. We calculate that 3×12=32 and we can evaluate the other products by using the power rule: 3×12×𝑥×𝑥×𝑦×𝑦=32𝑥×𝑥×𝑦×𝑦=32𝑥×𝑦=32𝑥𝑦.

It is also worth noting that this process shows us that the product of any monomials is always a monomial, since we add the integer exponents to get new integer exponents for the variables.

Example 5: Multiplying Monomials Involving More Than One Variable

Simplify 6𝑥𝑦×9𝑥𝑦.

Answer

We first rewrite the product, using the associativity and commutativity of multiplication, as 6𝑥𝑦×9𝑥𝑦=(6×(9))×𝑥×𝑥×𝑦×𝑦.

Now, we evaluate the product of the coefficients, 6×(9)=54, and the product of the variables using the product rule (𝑏×𝑏=𝑏): 𝑥×𝑥=𝑥=𝑥,𝑦×𝑦=𝑦=𝑦.

We get 6𝑥𝑦×9𝑥𝑦=54𝑥𝑦.

In our final example, we will simplify the product of monomials in multiple variables with fractional coefficients.

Example 6: Multiplying Monomials Involving More Than One Variable

Simplify 𝑥𝑦4×8𝑥𝑦5.

Answer

In this question, we want to multiply two monomials. To make the multiplication clearer, we can start by writing each monomial as a product of its coefficients and variables: 𝑥𝑦4×8𝑥𝑦5=14×𝑥×𝑦×85×𝑥×𝑦.

Now, we can use the commutativity and associativity of multiplication to rearrange our right-hand side expression: 14×𝑥×𝑦×85×𝑥×𝑦=14×85×𝑥×𝑥×𝑦×𝑦.

We can evaluate the product of the coefficients by multiplying the numerators and denominators: 14×85=1×(8)4×5=820=25.

We can evaluate the products of the variables by using the product rule (𝑏×𝑏=𝑏): 𝑥×𝑥=𝑥=𝑥,𝑦×𝑦=𝑦=𝑦.

We get 𝑥𝑦4×8𝑥𝑦5=2𝑥𝑦5.

Let’s finish by recapping some of the important points from this explainer.

Key Points

  • A monomial is a single algebraic term where every variable is raised to a nonnegative integer power.
  • The product rule for exponents tells us that for any rational number 𝑏 and nonnegative integers 𝑚 and 𝑛, we have 𝑏×𝑏=𝑏.
  • The product of monomials is a monomial.

Download the Nagwa Classes App

Attend sessions, chat with your teacher and class, and access class-specific questions. Download the Nagwa Classes app today!

Nagwa uses cookies to ensure you get the best experience on our website. Learn more about our Privacy Policy