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Mathematics LibreTexts

10.4: Divide Monomials (Part 1)

  • Page ID
    5015
  • [ "article:topic", "authorname:openstax" ]

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    Skills to Develop

    • Simplify expressions using the Quotient Property of Exponents
    • Simplify expressions with zero exponents
    • Simplify expressions using the Quotient to a Power Property
    • Simplify expressions by applying several properties
    • Divide monomials

    be prepared!

    Before you get started, take this readiness quiz.

    1. Simplify: \(\frac{8}{24}\). If you missed the problem, review Example 4.19.
    2. Simplify:(2m3)5. If you missed the problem, review Example 10.23.
    3. Simplify: \(\frac{12x}{12y}\). If you missed the problem, review Example 4.23.

    Simplify Expressions Using the Quotient Property of Exponents

    Earlier in this chapter, we developed the properties of exponents for multiplication. We summarize these properties here.

    Summary of Exponent Properties for Multiplication

    If a, b are real numbers and m, n are whole numbers, then

    Product Property am • an = am + n
    Power Property (am)n = am • n
    Product to a Power (ab)m = ambm

    Now we will look at the exponent properties for division. A quick memory refresher may help before we get started. In Fractions you learned that fractions may be simplified by dividing out common factors from the numerator and denominator using the Equivalent Fractions Property. This property will also help us work with algebraic fractions—which are also quotients.

    Definition: Equivalent Fractions Property

    If a, b, c are whole numbers where b ≠ 0, c ≠ 0, then

    $$\frac{a}{b} = \frac{a \cdot c}{b \cdot c}\quad and\quad \frac{a \cdot c}{b \cdot c} = \frac{a}{b}$$

    As before, we'll try to discover a property by looking at some examples.

    Consider $$\frac{x^{5}}{x^{2}}$$ and $$\frac{x^{2}}{x^{3}}$$
    What do they mean? $$\frac{x \cdot x \cdot x \cdot x \cdot x}{x \cdot x}$$   $$\frac{x \cdot x}{x \cdot x \cdot x}$$
    Use the Equivalent Fractions Property $$\frac{\cancel{x} \cdot \cancel{x} \cdot x \cdot x \cdot x}{\cancel{x} \cdot \cancel{x} \cdot 1}$$   $$\frac{\cancel{x} \cdot \cancel{x} \cdot 1}{\cancel{x} \cdot \cancel{x} \cdot x}$$
    Simplify. $$x^{3}$$   $$\frac{1}{x}$$

    Notice that in each case the bases were the same and we subtracted the exponents.

    • When the larger exponent was in the numerator, we were left with factors in the numerator and 1 in the denominator, which we simplified.
    • When the larger exponent was in the denominator, we were left with factors in the denominator, and 1 in the numerator, which could not be simplified.

    We write:

    $$\begin{split} \frac{x^{5}}{x^{2}} \qquad  &\quad \frac{x^{2}}{x^{3}} \\ x^{5-2} \qquad &\; \frac{1}{x^{3-2}} \\ x^{3} \qquad \quad &\quad \frac{1}{x} \end{split}$$

    Definition: Quotient Property of Exponents

    If a is a real number, a ≠ 0, and m, n are whole numbers, then

    $$\frac{a^{m}}{a^{n}} = a^{m-n}, \; m>n \quad and \quad \frac{a^{m}}{a^{n}} = \frac{1}{a^{n-m}},\; n>m$$

    A couple of examples with numbers may help to verify this property.

    $$\begin{split} \frac{3^{4}}{3^{2}} &\stackrel{?}{=} 3^{4-2} \qquad \; \frac{5^{2}}{5^{3}} \stackrel{?}{=} \frac{1}{5^{3-2}} \\ \frac{81}{9} &\stackrel{?}{=} 3^{2} \qquad \; \; \frac{25}{125} \stackrel{?}{=} \frac{1}{5^{1}} \\ 9 &= 9\; \checkmark \qquad \; \; \; \frac{1}{5} = \frac{1}{5}\; \checkmark \end{split}$$

    When we work with numbers and the exponent is less than or equal to 3, we will apply the exponent. When the exponent is greater than 3, we leave the answer in exponential form.

    Example 10.45:

    Simplify: (a) \(\frac{x^{10}}{x^{8}}\) (b) \(\frac{2^{9}}{2^{2}}\) 

    Solution

    To simplify an expression with a quotient, we need to first compare the exponents in the numerator and denominator.

    (a)

    Since 10 > 8, there are more factors of x in the numerator. $$\frac{x^{10}}{x^{8}}$$
    Use the quotient property with m > n, \(\frac{a^{m}}{a^{n}} = a^{m − n}\). $$x^{\textcolor{red}{10-8}}$$
    Simplify. $$x^{2}$$

    (b)

    Since 9 > 2, there are more factors of 2 in the numerator. $$\frac{2^{9}}{2^{2}}$$
    Use the quotient property with m > n, \(\frac{a^{m}}{a^{n}} = a^{m − n}\). $$2^{\textcolor{red}{9-2}}$$
    Simplify. $$2^{7}$$

    Notice that when the larger exponent is in the numerator, we are left with factors in the numerator.

    Exercise 10.89:

    Simplify: (a) \(\frac{x^{12}}{x^{9}}\) (b) \(\frac{7^{14}}{7^{5}}\) 

    Exercise 10.90:

    Simplify: (a) \(\frac{y^{23}}{y^{17}}\) (b) \(\frac{8^{15}}{8^{7}}\) 

    Example 10.46:

    Simplify: (a) \(\frac{b^{10}}{b^{15}}\) (b) \(\frac{3^{3}}{3^{5}}\)

    Solution

    To simplify an expression with a quotient, we need to first compare the exponents in the numerator and denominator.

    (a)

    Since 15 > 10, there are more factors of b in the denominator. $$\frac{b^{10}}{b^{15}}$$
    Use the quotient property with n > m, \(\frac{a^{m}}{a^{n}} = \frac{1}{a^{n − m}}\). $$\frac{\textcolor{red}{1}}{b^{\textcolor{red}{15-10}}}$$
    Simplify. $$\frac{1}{b^{5}}$$

    (b)

    Since 5 > 3, there are more factors of 3 in the denominator. $$\frac{3^{3}}{3^{5}}$$
    Use the quotient property with n > m, \(\frac{a^{m}}{a^{n}} = \frac{1}{a^{n − m}}\). $$\frac{\textcolor{red}{1}}{3^{\textcolor{red}{5-3}}}$$
    Simplify. $$\frac{1}{3^{2}}$$
    Apply the exponent. $$\frac{1}{9}$$

    Notice that when the larger exponent is in the denominator, we are left with factors in the denominator and 1 in the numerator.

    Exercise 10.91:

    Simplify: (a) \(\frac{x^{8}}{x^{15}}\) (b) \(\frac{12^{11}}{12^{21}}\)

    Exercise 10.92:

    Simplify: (a) \(\frac{m^{17}}{m^{26}}\) (b) \(\frac{7^{8}}{7^{14}}\)

    Example 10.47:

    Simplify: (a) \(\frac{a^{5}}{a^{9}}\) (b) \(\frac{x^{11}}{x^{7}}\)

    Solution

    (a)

    Since 9 > 5, there are more a's in the denominator and so we will end up with factors in the denominator. $$\frac{a^{5}}{a^{9}}$$
    Use the quotient property with n > m, \(\frac{a^{m}}{a^{n}} = \frac{1}{a^{n − m}}\). $$\frac{\textcolor{red}{1}}{a^{\textcolor{red}{9-5}}}$$
    Simplify. $$\frac{1}{a^{4}}$$

     (b)

    Notice there are more factors of x in the numerator, since 11 > 7. So we will end up with factors in the numerator. $$\frac{x^{11}}{x^{97}}$$
    Use the quotient property with m > n, \(\frac{a^{m}}{a^{n}} = a^{m − n}\). $$a^{\textcolor{red}{11-7}}$$
    Simplify. $$x^{4}$$

    Exercise 10.93:

    Simplify: (a) \(\frac{b^{19}}{b^{11}}\) (b) \(\frac{z^{5}}{z^{11}}\)

    Exercise 10.94:

    Simplify: (a) \(\frac{p^{9}}{p^{17}}\) (b) \(\frac{w^{13}}{w^{9}}\)

    Simplify Expressions with Zero Exponents

    A special case of the Quotient Property is when the exponents of the numerator and denominator are equal, such as an expression like \(\frac{a^{m}}{a^{m}}\). From earlier work with fractions, we know that

    $$\frac{2}{2} = 1 \qquad \frac{17}{17} = 1 \qquad \frac{-43}{-43} = 1$$

    In words, a number divided by itself is 1. So \(\frac{x}{x}\) = 1, for any x (x ≠ 0), since any number divided by itself is 1.

    The Quotient Property of Exponents shows us how to simplify \(\frac{a^{m}}{a^{n}}\) when m > n and when n < m by subtracting exponents. What if m = n?

    Now we will simplify \(\frac{a^{m}}{a^{m}}\) in two ways to lead us to the definition of the zero exponent. Consider first \(\frac{8}{8}\), which we know is 1.

      $$\frac{8}{8} = 1$$
    Write 8 as 23. $$\frac{2^{3}}{2^{3}} = 1$$
    Subtract exponents. $$2^{3-3} = 1$$
    Simplify. $$2^{0} = 1$$

    CNX_BMath_Figure_10_04_019_img.jpg

    We see \(\frac{a^{m}}{a^{n}}\) simplifies to a0 and to 1. So a0 = 1.

    Definition: Zero Exponent

    If a is a non-zero number, then a0 = 1. Any nonzero number raised to the zero power is 1.

    In this text, we assume any variable that we raise to the zero power is not zero.

    Example 10.48:

    Simplify: (a) 120 (b) y0

    Solution

    The definition says any non-zero number raised to the zero power is 1.

    (a) 120

    Use the definition of the zero exponent. 1

    (b) y0

    Use the definition of the zero exponent. 1

    Exercise 10.95:

    Simplify: (a) 170 (b) m0

    Exercise 10.96:

    Simplify: (a) k0 (b) 290

    Now that we have defined the zero exponent, we can expand all the Properties of Exponents to include whole number exponents.

    What about raising an expression to the zero power? Let's look at (2x)0. We can use the product to a power rule to rewrite this expression.

      (2x)0
    Use the Product to a Power Rule. 20x0
    Use the Zero Exponent Property. 1 • 1
    Simplify. 1

    This tells us that any non-zero expression raised to the zero power is one.

    Example 10.49:

    Simplify: (7z)0.

    Solution
    Use the definition of the zero exponent. 1

    Exercise 10.97:

    Simplify: (−4y)0.

    Exercise 10.98:

    Simplify:  \(\left(\dfrac{2}{3} x\right)^{0}\).

    Example 10.50:

    Simplify: (a) (−3x2y)0 (b) −3x2y0

    Solution

    (a) (−3x2y)0

    The product is raised to the zero power. (−3x2y)0
    Use the definition of the zero exponent. 1

    (b) −3x2y0

    Notice that only the variable y is being raised to the zero power. −3x2y0
    Use the definition of the zero exponent. −3x• 1 
    Simplify. −3x2

    Exercise 10.99:

    Simplify: (a) (7x2y)0 (b) 7x2y0

    Exercise 10.100:

    Simplify: (a) −23x2y0 (b) (−23x2y)0

    Simplify Expressions Using the Quotient to a Power Property

    Now we will look at an example that will lead us to the Quotient to a Power Property.

      $$\left(\dfrac{x}{y}\right)^{3}$$
    This means $$\frac{x}{y} \cdot \frac{x}{y} \cdot \frac{x}{y}$$
    Multiply the fractions. $$\frac{x \cdot x \cdot x}{y \cdot y \cdot y}$$
    Write with exponents. $$\frac{x^{3}}{y^{3}}$$

    Notice that the exponent applies to both the numerator and the denominator. We see that \(\left(\dfrac{x}{y}\right)^{3}\) is \(\frac{x^{3}}{y^{3}}\). We write:

    $$\left(\dfrac{x}{y}\right)^{3} = \frac{x^{3}}{y^{3}}$$

    This leads to the Quotient to a Power Property for Exponents.

    Definition: Quotient to a Power Property of Exponents

    If a and b are real numbers, b ≠ 0, and m is a counting number, then

    $$\left(\dfrac{a}{b}\right)^{m} = \frac{a^{m}}{b^{m}}$$

    To raise a fraction to a power, raise the numerator and denominator to that power.

    An example with numbers may help you understand this property:

    $$\begin{split} \left(\dfrac{2}{3}\right)^{3} &\stackrel{?}{=} \frac{2^{3}}{3^{3}} \\ \frac{2}{3} \cdot \frac{2}{3} \cdot \frac{2}{3} &\stackrel{?}{=} \frac{8}{27} \\ \frac{8}{27} &= \frac{8}{27}\; \checkmark \end{split}$$

    Example 10.51:

    Simplify: (a) \(\left(\dfrac{5}{8}\right)^{2}\) (b) \(\left(\dfrac{x}{3}\right)^{4}\) (c) \(\left(\dfrac{y}{m}\right)^{3}\)

    Solution

    (a) \(\left(\dfrac{5}{8}\right)^{2}\)

    Use the Quotient to a Power Property, \(\left(\dfrac{a}{b}\right)^{m} = \frac{a^{m}}{b^{m}}\). $$\frac{5^{\textcolor{red}{2}}}{8^{\textcolor{red}{2}}}$$
    Simplify. $$\frac{25}{64}$$

    (b) \(\left(\dfrac{x}{3}\right)^{4}\)

    Use the Quotient to a Power Property, \(\left(\dfrac{a}{b}\right)^{m} = \frac{a^{m}}{b^{m}}\). $$\frac{x^{\textcolor{red}{4}}}{3^{\textcolor{red}{4}}}$$
    Simplify. $$\frac{x^{4}}{81}$$

    (c) \(\left(\dfrac{y}{m}\right)^{3}\)

    Raise the numerator and denominator to the third power. $$\frac{y^{\textcolor{red}{3}}}{m^{\textcolor{red}{3}}}$$

    Exercise 10.101:

    Simplify: (a) \(\left(\dfrac{7}{9}\right)^{2}\) (b) \(\left(\dfrac{y}{8}\right)^{3}\) (c) \(\left(\dfrac{p}{q}\right)^{6}\)

    Exercise 10.102:

    Simplify: (a) \(\left(\dfrac{1}{8}\right)^{2}\) (b) \(\left(\dfrac{-5}{m}\right)^{3}\) (c) \(\left(\dfrac{r}{s}\right)^{4}\)

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