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Study Guides > Intermediate Algebra

Read: Rationalize Denominators

Learning Objectives

  • Rationalize a denominator containing a radical expression
    • Define and recognize a rational number
    • Rationalize denominators with one or multiple terms
Although radicals follow the same rules that integers do, it is often difficult to figure out the value of an expression containing radicals. For example, you probably have a good sense of how much 48, 0.75 \frac{4}{8},\ 0.75 and 69 \frac{6}{9} are, but what about the quantities 12 \frac{1}{\sqrt{2}} and 15 \frac{1}{\sqrt{5}}? These are much harder to visualize. That said, sometimes you have to work with expressions that contain many radicals. Often the value of these expressions is not immediately clear. In cases where you have a fraction with a radical in the denominator, you can use a technique called rationalizing a denominator to eliminate the radical. The point of rationalizing a denominator is to make it easier to understand what the quantity really is by removing radicals from the denominators. The idea of rationalizing a denominator makes a bit more sense if you consider the definition of “rationalize.” Recall that the numbers 55, 12 \frac{1}{2}, and 0.75 0.75 are all known as rational numbers—they can each be expressed as a ratio of two integers (51,12 \frac{5}{1},\frac{1}{2}, and 34 \frac{3}{4} respectively). Some radicals are irrational numbers because they cannot be represented as a ratio of two integers. As a result, the point of rationalizing a denominator is to change the expression so that the denominator becomes a rational number. Here are some examples of irrational and rational denominators.

Irrational

Rational

12 \frac{1}{\sqrt{2}}

=

22 \frac{\sqrt{2}}{2}

2+33 \frac{2+\sqrt{3}}{\sqrt{3}}

=

23+33 \frac{2\sqrt{3}+3}{3}

Now let’s examine how to get from irrational to rational denominators.

Rationalizing Denominators with One Term

Let’s start with the fraction 12 \frac{1}{\sqrt{2}}. Its denominator is 2 \sqrt{2}, an irrational number. This makes it difficult to figure out what the value of 12 \frac{1}{\sqrt{2}} is. You can rename this fraction without changing its value, if you multiply it by 11. In this case, set 11 equal to 22 \frac{\sqrt{2}}{\sqrt{2}}. Watch what happens.

121=1222=222=24=22 \frac{1}{\sqrt{2}}\cdot 1=\frac{1}{\sqrt{2}}\cdot \frac{\sqrt{2}}{\sqrt{2}}=\frac{\sqrt{2}}{\sqrt{2\cdot 2}}=\frac{\sqrt{2}}{\sqrt{4}}=\frac{\sqrt{2}}{2}

The denominator of the new fraction is no longer a radical (notice, however, that the numerator is). So why choose to multiply 12 \frac{1}{\sqrt{2}} by 22 \frac{\sqrt{2}}{\sqrt{2}}? You knew that the square root of a number times itself will be a whole number. In algebraic terms, this idea is represented by xx=x \sqrt{x}\cdot \sqrt{x}=x. Look back to the denominators in the multiplication of 121 \frac{1}{\sqrt{2}}\cdot 1. Do you see where 22=4=2 \sqrt{2}\cdot \sqrt{2}=\sqrt{4}=2? In the following videos we show examples of rationalizing the denominator of a radical expression that contains integer radicands. https://youtu.be/K7NdhPLVl7g   Here are some more examples. Notice how the value of the fraction is not changed at all—it is simply being multiplied by another name for 11.

Example

Rationalize the denominator.

2+33 \frac{2+\sqrt{3}}{\sqrt{3}}

Answer: The denominator of this fraction is 3 \sqrt{3}. To make it into a rational number, multiply it by 3 \sqrt{3}, since 33=3 \sqrt{3}\cdot \sqrt{3}=3.

2+33 \frac{2+\sqrt{3}}{\sqrt{3}}

Multiply the entire fraction by another name for 11, 33 \frac{\sqrt{3}}{\sqrt{3}}.

2+33333(2+3)33\begin{array}{r}\frac{2+\sqrt{3}}{\sqrt{3}}\cdot \frac{\sqrt{3}}{\sqrt{3}}\\\\\frac{\sqrt{3}(2+\sqrt{3})}{\sqrt{3}\cdot \sqrt{3}}\end{array}

Use the Distributive Property to multiply 3(2+3) \sqrt{3}(2+\sqrt{3}).

23+339 \frac{2\sqrt{3}+\sqrt{3}\cdot \sqrt{3}}{\sqrt{9}}

Simplify the radicals, where possible. 9=3 \sqrt{9}=3.

23+99 \frac{2\sqrt{3}+\sqrt{9}}{\sqrt{9}}

Answer

2+33=23+33 \frac{2+\sqrt{3}}{\sqrt{3}}=\frac{2\sqrt{3}+3}{3}

You can use the same method to rationalize denominators to simplify fractions with radicals that contain a variable. As long as you multiply the original expression by another name for 11, you can eliminate a radical in the denominator without changing the value of the expression itself.

Example

Rationalize the denominator.

x+yx, where x0 \frac{\sqrt{x}+\sqrt{y}}{\sqrt{x}},\text{ where }x\ne \text{0}

Answer: The denominator is x \sqrt{x}, so the entire expression can be multiplied by xx \frac{\sqrt{x}}{\sqrt{x}} to get rid of the radical in the denominator.

x+yxxxx(x+y)xx \begin{array}{c}\frac{\sqrt{x}+\sqrt{y}}{\sqrt{x}}\cdot \frac{\sqrt{x}}{\sqrt{x}}\\\\\frac{\sqrt{x}(\sqrt{x}+\sqrt{y})}{\sqrt{x}\cdot \sqrt{x}}\end{array}

Use the Distributive Property. Simplify the radicals, where possible. Remember that xx=x \sqrt{x}\cdot \sqrt{x}=x.

xx+xyxx \frac{\sqrt{x}\cdot \sqrt{x}+\sqrt{x}\cdot \sqrt{y}}{\sqrt{x}\cdot \sqrt{x}}

Answer

x+yx=x+xyx \frac{\sqrt{x}+\sqrt{y}}{\sqrt{x}}=\frac{x+\sqrt{xy}}{x}

Example

Rationalize the denominator and simplify.

100x11y, where y0 \sqrt{\frac{100x}{11y}},\text{ where }y\ne \text{0}

Answer: Rewrite ab \sqrt{\frac{a}{b}} as ab \frac{\sqrt{a}}{\sqrt{b}}.

100x11y \frac{\sqrt{100x}}{\sqrt{11y}}

The denominator is 11y \sqrt{11y}, so multiplying the entire expression by 11y11y \frac{\sqrt{11y}}{\sqrt{11y}} will rationalize the denominator.

100x11y11y11y \frac{\sqrt{100x\cdot11y}}{\sqrt{11y}\cdot\sqrt{11y}}

Multiply and simplify the radicals, where possible.

10011xy11y11y \frac{\sqrt{100\cdot 11xy}}{\sqrt{11y}\cdot \sqrt{11y}}

100 is a perfect square. Remember that100=10 \sqrt{100}=10 and xx=x \sqrt{x}\cdot \sqrt{x}=x.

10011xy11y11y \frac{\sqrt{100}\cdot \sqrt{11xy}}{\sqrt{11y}\cdot \sqrt{11y}}

Answer

100x11y=1011xy11y \sqrt{\frac{100x}{11y}}=\frac{10\sqrt{11xy}}{11y}

Rationalizing Denominators with Two Terms

Denominators do not always contain just one term, as shown in the previous examples. Sometimes, you will see expressions like 32+3 \frac{3}{\sqrt{2}+3} where the denominator is composed of two terms, 2 \sqrt{2} and +3+3. Unfortunately, you cannot rationalize these denominators the same way you rationalize single-term denominators. If you multiply 2+3 \sqrt{2}+3 by 2 \sqrt{2}, you get 2+32 2+3\sqrt{2}. The original 2 \sqrt{2} is gone, but now the quantity 32 3\sqrt{2} has appeared...this is no better! In order to rationalize this denominator, you want to square the radical term and somehow prevent the integer term from being multiplied by a radical. Is this possible? It is possible—and you have already seen how to do it! Recall that when binomials of the form (a+b)(ab) (a+b)(a-b) are multiplied, the product is . So, for example, (x+3)(x3)=x23x+3x9=x29 (x+3)(x-3)={{x}^{2}}-3x+3x-9={{x}^{2}}-9; notice that the terms 3x−3x and +3x+3x combine to 0. Now for the connection to rationalizing denominators: what if you replaced x with 2 \sqrt{2}? Look at the side by side examples below. Just as 3x+3x -3x+3x combines to 00 on the left, 32+32 -3\sqrt{2}+3\sqrt{2} combines to 00 on the right.
(x+3)(x3)=x23x+3x9=x29 \begin{array}{l}(x+3)(x-3)\\={{x}^{2}}-3x+3x-9\\={{x}^{2}}-9\end{array} (2+3)(23)=(2)232+329=(2)29=29=7 \begin{array}{l}\left( \sqrt{2}+3 \right)\left( \sqrt{2}-3 \right)\\={{\left( \sqrt{2} \right)}^{2}}-3\sqrt{2}+3\sqrt{2}-9\\={{\left( \sqrt{2} \right)}^{2}}-9\\=2-9\\=-7\end{array}
There you have it! Multiplying 2+3 \sqrt{2}+3 by 23 \sqrt{2}-3 removed one radical without adding another. In this example, 23 \sqrt{2}-3 is known as a conjugate, and 2+3 \sqrt{2}+3 and 23 \sqrt{2}-3 are known as a conjugate pair. To find the conjugate of a binomial that includes radicals, change the sign of the second term to its opposite as shown in the table below.
Term Conjugate Product
2+3 \sqrt{2}+3 23 \sqrt{2}-3 (2+3)(23)=(2)2(3)2=29=7 \left( \sqrt{2}+3 \right)\left( \sqrt{2}-3 \right)={{\left( \sqrt{2} \right)}^{2}}-{{\left( 3 \right)}^{2}}=2-9=-7
x5 \sqrt{x}-5 x+5 \sqrt{x}+5 (x5)(x+5)=(x)2(5)2=x25 \left( \sqrt{x}-5 \right)\left( \sqrt{x}+5 \right)={{\left( \sqrt{x} \right)}^{2}}-{{\left( 5 \right)}^{2}}=x-25
82x 8-2\sqrt{x} 8+2x 8+2\sqrt{x} (82x)(8+2x)=(8)2(2x)2=644x \left( 8-2\sqrt{x} \right)\left( 8+2\sqrt{x} \right)={{\left( 8 \right)}^{2}}-{{\left( 2\sqrt{x} \right)}^{2}}=64-4x
1+xy 1+\sqrt{xy} 1xy 1-\sqrt{xy} (1+xy)(1xy)=(1)2(xy)2=1xy \left( 1+\sqrt{xy} \right)\left( 1-\sqrt{xy} \right)={{\left( 1 \right)}^{2}}-{{\left( \sqrt{xy} \right)}^{2}}=1-xy

Example

Rationalize the denominator and simplify.

573+5 \frac{5-\sqrt{7}}{3+\sqrt{5}}

Answer: Find the conjugate of 3+5 3+\sqrt{5}. Then multiply the entire expression by 3535 \frac{3-\sqrt{5}}{3-\sqrt{5}}.

573+53535(57)(35)(3+5)(35) \begin{array}{c}\frac{5-\sqrt{7}}{3+\sqrt{5}}\cdot \frac{3-\sqrt{5}}{3-\sqrt{5}}\\\\\frac{\left( 5-\sqrt{7} \right)\left( 3-\sqrt{5} \right)}{\left( 3+\sqrt{5} \right)\left( 3-\sqrt{5} \right)}\end{array}

Use the Distributive Property to multiply the binomials in the numerator and denominator.

535537+753335+3555 \frac{5\cdot 3-5\sqrt{5}-3\sqrt{7}+\sqrt{7}\cdot \sqrt{5}}{3\cdot 3-3\sqrt{5}+3\sqrt{5}-\sqrt{5}\cdot \sqrt{5}}

Since you multiplied by the conjugate of the denominator, the radical terms in the denominator will combine to 00.

155537+35935+3525 \frac{15-5\sqrt{5}-3\sqrt{7}+\sqrt{35}}{9-3\sqrt{5}+3\sqrt{5}-\sqrt{25}}

Simplify radicals where possible.

155537+35925155537+3595 \begin{array}{c}\frac{15-5\sqrt{5}-3\sqrt{7}+\sqrt{35}}{9-\sqrt{25}}\\\\\frac{15-5\sqrt{5}-3\sqrt{7}+\sqrt{35}}{9-5}\end{array}

Answer

573+5=155537+354 \frac{5-\sqrt{7}}{3+\sqrt{5}}=\frac{15-5\sqrt{5}-3\sqrt{7}+\sqrt{35}}{4}

Example

Rationalize the denominator and simplify.

xx+2 \frac{\sqrt{x}}{\sqrt{x}+2}

Answer: Find the conjugate of x+2 \sqrt{x}+2. Then multiply the numerator and denominator by x2x2 \frac{\sqrt{x}-2}{\sqrt{x}-2}.

xx+2x2x2x(x2)(x+2)(x2) \begin{array}{c}\frac{\sqrt{x}}{\sqrt{x}+2}\cdot \frac{\sqrt{x}-2}{\sqrt{x}-2}\\\\\frac{\sqrt{x}\left( \sqrt{x}-2 \right)}{\left( \sqrt{x}+2 \right)\left( \sqrt{x}-2 \right)}\end{array}

Use the Distributive Property to multiply the binomials in the numerator and denominator.

xx2xxx2x+2x22 \frac{\sqrt{x}\cdot \sqrt{x}-2\sqrt{x}}{\sqrt{x}\cdot \sqrt{x}-2\sqrt{x}+2\sqrt{x}-2\cdot 2}

Simplify. Remember that xx=x \sqrt{x}\cdot \sqrt{x}=x. Since you multiplied by the conjugate of the denominator, the radical terms in the denominator will combine to 00.

xx2xxx2x+2x4 \frac{\sqrt{x}\cdot \sqrt{x}-2\sqrt{x}}{\sqrt{x}\cdot \sqrt{x}-2\sqrt{x}+2\sqrt{x}-4}

Answer

xx+2=x2xx4 \frac{\sqrt{x}}{\sqrt{x}+2}=\frac{x-2\sqrt{x}}{x-4}

One word of caution: this method will work for binomials that include a square root, but not for binomials with roots greater than 22. This is because squaring a root that has an index greater than 2 does not remove the root, as shown below.

(103+5)(1035)=(103)25103+510325=(103)225=100325 \begin{array}{l}\left( \sqrt[3]{10}+5 \right)\left( \sqrt[3]{10}-5 \right)\\={{\left( \sqrt[3]{10} \right)}^{2}}-5\sqrt[3]{10}+5\sqrt[3]{10}-25\\={{\left( \sqrt[3]{10} \right)}^{2}}-25\\=\sqrt[3]{100}-25\end{array}

1003 \sqrt[3]{100} cannot be simplified any further, since its prime factors are 2255 2\cdot 2\cdot 5\cdot 5. There are no cubed numbers to pull out! Multiplying 103+5 \sqrt[3]{10}+5 by its conjugate does not result in a radical-free expression. In the following video we show more examples of how to rationalize a denominator using the conjugate. https://youtu.be/vINRIRgeKqU

Summary

When you encounter a fraction that contains a radical in the denominator, you can eliminate the radical by using a process called rationalizing the denominator. To rationalize a denominator, you need to find a quantity that, when multiplied by the denominator, will create a rational number (no radical terms) in the denominator. When the denominator contains a single term, as in 15 \frac{1}{\sqrt{5}}, multiplying the fraction by 55 \frac{\sqrt{5}}{\sqrt{5}} will remove the radical from the denominator. When the denominator contains two terms, as in25+3 \frac{2}{\sqrt{5}+3}, identify the conjugate of the denominator, here53 \sqrt{5}-3, and multiply both numerator and denominator by the conjugate.

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