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7.2: Rational Exponents and Roots

Difficulty Level: At Grade Created by: CK-12
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A planet's maximum distance from the sun (in astronomical units) is given by the formula d = p^{\frac {2}{3}} , were p is the period (in years) of the planet's orbit around the sun. If a planet's orbit around the sun is 27 years, what is its distance from the sun?

Guidance

Now that you are familiar with nth roots, we will convert them into exponents. Let’s look at the square root and see if we can use the properties of exponents to determine what exponential number it is equivalent to.

Investigation: Writing the Square Root as an Exponent

1. Evaluate \left(\sqrt{x}\right)^2 . What happens?

  • The \sqrt{\;\;} and the ^2 cancel each other out, \left(\sqrt{x}^2\right)=x .

2. Recall that when a power is raised to another power, we multiply the exponents. Therefore, we can rewrite the exponents and root as an equation, n\cdot 2=1 . Solve for n .

  • \frac{n \cdot \cancel{2}}{\cancel{2}}&=\frac{1}{2} \\n&=\frac{1}{2}

3. From #2, we can conclude that \sqrt{\;\;}=\frac{1}{2} .

  • \left(\sqrt{x}\right)^2=\left(x^{\frac{1}{2}}\right)^2=x^{\left(\frac{1}{2}\right)\cdot 2}=x^1=x

From this investigation, we see that \sqrt{x}=x^{\frac{1}{2}} . We can extend this idea to the other roots as well; \sqrt[3]{x}=x^{\frac{1}{3}}=\sqrt[4]{x}=x^{\frac{1}{4}},\ldots \sqrt[n]{x}=x^{\frac{1}{n}} .

Example A

Find 256^{\frac{1}{4}} .

Solution: Rewrite this expression in terms of roots. A number to the one-fourth power is the same as the fourth root.

256^{\frac{1}{4}}=\sqrt[4]{256}=\sqrt[4]{4^4}=4

Therefore, 256^{\frac{1}{4}}=4 .

Example B

Find 49^{\frac{3}{2}} .

Solution: This problem is the same as the ones in the previous concept. However, now, the root is written in the exponent. Rewrite the problem.

49^{\frac{3}{2}}=\left(49^3\right)^{\frac{1}{2}}=\sqrt{49^3} or \left(\sqrt{49}\right)^3

From the previous concept, we know that it is easier to evaluate the second option above. \left(\sqrt{49}\right)^3=7^3=343 .

The Rational Exponent Theorem: For any real number a , root n , and exponent m , the following is always true: a^{\frac{m}{n}}=\sqrt[n]{a^m}=\left(\sqrt[n]{a}\right)^m .

Example C

Find 5^{\frac{2}{3}} using a calculator. Round your answer to the nearest hundredth.

Solution: To type this into a calculator, the keystrokes would probably look like: 5^{\frac{2}{3}} . The “^” symbol is used to indicate a power. Anything in parenthesis after the “^” would be in the exponent. Evaluating this, we have 2.924017738..., or just 2.92.

Other calculators might have a x^y button. This button has the same purpose as the ^ and would be used in the exact same way.

Intro Problem Revisit Substitute 27 for p and solve.

d = 27^{\frac{2}{3}}

Rewrite the problem.

27^{\frac{2}{3}}=\left(27^2\right)^{\frac{1}{3}}=\sqrt[3]{27^2} or \sqrt[3]{27}^2

\left(\sqrt[3]{27}\right)^2=3^2=9 .

Therefore, the planet's distance from the sun is 9 astronomical units.

Guided Practice

1. Rewrite \sqrt[7]{12} using rational exponents. Then, use a calculator to find the answer.

2. Rewrite 845^{\frac{4}{9}} using roots. Then, use a calculator to find the answer.

Evaluate without a calculator.

3. 125^{\frac{4}{3}}

4. 256^{\frac{5}{8}}

5. \sqrt{81^{\frac{1}{2}}}

Answers

1. Using rational exponents, the 7^{th} root becomes the \frac{1}{7} power; 12^{\frac{1}{7}}=1.426 .

2. Using roots, the 9 in the denominator of the exponent is the root; \sqrt[9]{845^4}=19.99 . To enter this into a calculator, you can use the rational exponents. If you have a TI-83 or 84, press MATH and select 5 : \sqrt[x]{\;\;} . On the screen, you should type 9\sqrt[x]{\;\;} \ 845^\land 4 to get the correct answer. You can also enter 845^\land {\left(\frac{4}{9}\right)} and get the exact same answer

3.  125^{\frac{4}{3}}=\left(\sqrt[3]{125}\right)^4=5^4=625

4. 256^{\frac{5}{8}}=\left(\sqrt[8]{256}\right)^5=2^5=32

5. \sqrt{81^{\frac{1}{2}}}=\sqrt{\sqrt{81}}=\sqrt{9}=3

Vocabulary

Rational Exponent
An exponent that can be written as a fraction. For any n^{th} root, the n of the root can be written in the denominator of a rational exponent. \sqrt[{\color{red}n}]{x}=x^{\frac{1}{{\color{red}n}}} .

Explore More

Write the following expressions using rational exponents and then evaluate using a calculator. Answers should be rounded to the nearest hundredth.

  1. \sqrt[5]{45}
  2. \sqrt[9]{140}
  3. \sqrt[8]{50}^3

Write the following expressions using roots and then evaluate using a calculator. Answers should be rounded to the nearest hundredth.

  1. 72^{\frac{5}{3}}
  2. 95^{\frac{2}{3}}
  3. 125^{\frac{3}{4}}

Evaluate the following without a calculator.

  1. 64^{\frac{2}{3}}
  2. 27^{\frac{4}{3}}
  3. 16^{\frac{5}{4}}
  4. \sqrt{25^3}
  5. \sqrt[2]{9}^5
  6. \sqrt[5]{32^2}

For the following problems, rewrite the expressions with rational exponents and then simplify the exponent and evaluate without a calculator.

  1. \sqrt[4]{\left(\frac{2}{3}\right)^8}
  2. \sqrt[3]{\frac{7}{2}}^6
  3. \sqrt{\left(16\right)^{\frac{1}{2}}}^6

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At Grade

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Date Created:

Mar 12, 2013

Last Modified:

Oct 28, 2014
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