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8.5: Negative Exponents

Difficulty Level: At Grade Created by: CK-12
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Practice Negative Exponents
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What if you had a mathematical expression like x2x6\begin{align*}\frac{x^{-2}}{x^{-6}}\end{align*} that contained negative exponents? How could you simplify it so that none of the exponents were negative? After completing this Concept, you'll be able to simplify expressions with negative exponents like this one.

Guidance

The product and quotient rules for exponents lead to many interesting concepts. For example, so far we’ve mostly just considered positive, whole numbers as exponents, but you might be wondering what happens when the exponent isn’t a positive whole number. What does it mean to raise something to the power of zero, or -1, or 12\begin{align*}\frac{1}{2}\end{align*}? In this lesson, we’ll find out.

Simplify Expressions With Negative Exponents

When we learned the quotient rule for exponents (xnxm=x(nm))\begin{align*}\left ( \frac{x^n}{x^m}=x^{(n-m)} \right )\end{align*}, we saw that it applies even when the exponent in the denominator is bigger than the one in the numerator. Canceling out the factors in the numerator and denominator leaves the leftover factors in the denominator, and subtracting the exponents leaves a negative number. So negative exponents simply represent fractions with exponents in the denominator. This can be summarized in a rule:

Negative Power Rule for Exponents: xn=1xn\begin{align*}x^{-n} = \frac{1}{x^n}\end{align*}, where x0\begin{align*}x \neq 0\end{align*}

Negative exponents can be applied to products and quotients also. Here’s an example of a negative exponent being applied to a product:

(x3y)2=x6y2x6y2=1x61y2=1x6y2using the power ruleusing the negative power rule separately on each variable

And here’s one applied to a quotient:

(ab)3=a3b3a3b3=a311b3=1a3b311a3b31=b3a3b3a3=(ba)3using the power rule for quotientsusing the negative power rule on each variable separatelysimplifying the division of fractionsusing the power rule for quotients in reverse.

That last step wasn’t really necessary, but putting the answer in that form shows us something useful: (ab)3\begin{align*}\left ( \frac{a}{b} \right )^{-3}\end{align*} is equal to (ba)3\begin{align*}\left ( \frac{b}{a} \right )^3\end{align*}. This is an example of a rule we can apply more generally:

Negative Power Rule for Fractions: (xy)n=(yx)n,\begin{align*}\left ( \frac{x}{y} \right )^{-n} = \left ( \frac{y}{x} \right )^n,\end{align*} where x0,y0\begin{align*}x \neq 0, y \neq 0\end{align*}

This rule can be useful when you want to write out an expression without using fractions.

Example A

Write the following expressions without fractions.

a) 1x\begin{align*}\frac{1}{x}\end{align*}

b) 2x2\begin{align*}\frac{2}{x^2}\end{align*}

c) x2y3\begin{align*}\frac{x^2}{y^3}\end{align*}

d) 3xy\begin{align*}\frac{3}{xy}\end{align*}

Solution

a) 1x=x1\begin{align*}\frac{1}{x} = x^{-1}\end{align*}

b) 2x2=2x2\begin{align*}\frac{2}{x^2} = 2x^{-2}\end{align*}

c) x2y3=x2y3\begin{align*}\frac{x^2}{y^3} = x^2y^{-3}\end{align*}

d) 3xy=3x1y1\begin{align*}\frac{3}{xy} = 3x^{-1}y^{-1}\end{align*}

Example B

Simplify the following expressions and write them without fractions.

a) 4a2b32a5b\begin{align*}\frac{4a^2b^3}{2a^5b}\end{align*}

b) (x3y2)3x2y4\begin{align*}\left ( \frac{x}{3y^2} \right )^3 \cdot \frac{x^2y}{4}\end{align*}

Solution

a) Reduce the numbers and apply the quotient rule to each variable separately:

4a2b32a5b=2a25b31=2a3b2

b) Apply the power rule for quotients first:

(2xy2)3x2y4=8x3y6x2y4

Then simplify the numbers, and use the product rule on the x\begin{align*}x\end{align*}’s and the quotient rule on the y\begin{align*}y\end{align*}’s:

8x3y6x2y4=2x3+2y16=2x5y5

You can also use the negative power rule the other way around if you want to write an expression without negative exponents.

Example C

Write the following expressions without negative exponents.

a) 3x3\begin{align*}3x^{-3}\end{align*}

b) a2b3c1\begin{align*}a^2b^{-3}c^{-1}\end{align*}

c) 4x1y3\begin{align*}4x^{-1}y^3\end{align*}

d) 2x2y3\begin{align*}\frac{2x^{-2}}{y^{-3}}\end{align*}

Solution

a) 3x3=3x3\begin{align*}3x^{-3} = \frac{3}{x^3}\end{align*}

b) a2b3c1=a2b3c\begin{align*}a^2b^{-3}c^{-1} = \frac{a^2}{b^3c}\end{align*}

c) 4x1y3=4y3x\begin{align*}4x^{-1}y^3 = \frac{4y^3}{x}\end{align*}

d) 2x2y3=2y3x2\begin{align*}\frac{2x^{-2}}{y^{-3}} = \frac{2y^3}{x^2}\end{align*}

Watch this video for help with the Examples above.

Vocabulary

• Negative Power Rule for Exponents: xn=1xn\begin{align*}x^{-n} = \frac{1}{x^n}\end{align*}, where x0.\begin{align*}x \neq 0.\end{align*}

Guided Practice

Simplify the following expressions and write the answers without negative powers.

a) (ab2b3)2\begin{align*}\left ( \frac{ab^{-2}}{b^3} \right )^2\end{align*}

b) x3y2x2y2\begin{align*}\frac{x^{-3}y^2}{x^2y^{-2}}\end{align*}

Solution

a) Apply the quotient rule inside the parentheses: (ab2b3)2=(ab5)2\begin{align*}\left ( \frac{ab^{-2}}{b^3} \right )^2 = (ab^{-5})^2\end{align*}

Then apply the power rule: (ab5)2=a2b10=a2b10\begin{align*}(ab^{-5})^2 = a^2b^{-10} = \frac{a^2}{b^{10}}\end{align*}

b) Apply the quotient rule to each variable separately: x3y2x2y2=x32y2(2)=x5y4=y4x5\begin{align*}\frac{x^{-3}y^2}{x^2y^{-2}} = x^{-3-2}y^{2-(-2)} = x^{-5}y^4 = \frac{y^4}{x^5}\end{align*}

Practice

Simplify the following expressions in such a way that there aren't any negative exponents in the answer.

1. x1y2\begin{align*}x^{-1}y^2\end{align*}
2. x4\begin{align*}x^{-4}\end{align*}
3. x3x7\begin{align*}\frac{x^{-3}}{x^{-7}}\end{align*}
4. x3y5z7\begin{align*}\frac{x^{-3}y^{-5}}{z^{-7}}\end{align*}
5. (ab)2\begin{align*}\left ( \frac{a}{b} \right )^{-2}\end{align*}
6. (3a2b2c3)3\begin{align*}(3a^{-2}b^2c^3)^3\end{align*}

Simplify the following expressions in such a way that there aren't any fractions in the answer.

1. a3(a5)a6\begin{align*}\frac{a^{-3}(a^5)}{a^{-6}}\end{align*}
2. 5x6y2x8y\begin{align*}\frac{5x^6y^2}{x^8y}\end{align*}
3. (4ab6)3(ab)5\begin{align*}\frac{(4ab^6)^3}{(ab)^5}\end{align*}
4. (3x3)(4x4)(2y)2\begin{align*}\frac{(3x^3)(4x^4)}{(2y)^2}\end{align*}
5. a2b3c1\begin{align*}\frac{a^{-2}b^{-3}}{c^{-1}}\end{align*}

Vocabulary Language: English

Negative Exponent Property

Negative Exponent Property

The negative exponent property states that $\frac{1}{a^m} = a^{-m}$ and $\frac{1}{a^{-m}} = a^m$ for $a \neq 0$.
quotient rule

quotient rule

In calculus, the quotient rule states that if $f$ and $g$ are differentiable functions at $x$ and $g(x) \ne 0$, then $\frac {d}{dx}\left [ \frac{f(x)}{g(x)} \right ]= \frac {g(x) \frac {d}{dx}\left [{f(x)} \right ] - f(x) \frac{d}{dx} \left [{g(x)} \right ]}{\left [{g(x)} \right ]^2}$.
Zero Exponent Property

Zero Exponent Property

The zero exponent property says that for all $a \neq 0$, $a^0 = 1$.

Aug 13, 2012

Dec 15, 2015