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Rationalize a two-term denominator

When the denominator of a fraction is a sum or difference with square roots, we use the Product of Conjugates pattern to rationalize the denominator.

( a b ) ( a + b ) ( 2 5 ) ( 2 + 5 ) a 2 b 2 2 2 ( 5 ) 2 4 5 −1

When we multiply a binomial that includes a square root by its conjugate, the product has no square roots.

Simplify: 4 4 + 2 .

Solution

.
Multiply the numerator and denominator by the conjugate of the denominator. .
Multiply the conjugates in the denominator. .
Simplify the denominator. .
Simplify the denominator. .
Remove common factors from the numerator and denominator. .
We leave the numerator in factored form to make it easier to look for common factors after we have simplified the denominator.

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Simplify: 2 2 + 3 .

2 ( 2 3 ) 1

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Simplify: 5 5 + 3 .

5 ( 5 3 ) 22

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Simplify: 5 2 3 .

Solution

.
Multiply the numerator and denominator by the conjugate of the denominator. .
Multiply the conjugates in the denominator. .
Simplify the denominator. .
Simplify the denominator. .
Simplify. .

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Simplify: 3 1 5 .

3 ( 1 + 5 ) 4

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Simplify: 2 4 6 .

4 + 6 5

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Simplify: 3 u 6 .

Solution

.
Multiply the numerator and denominator by the conjugate of the denominator. .
Multiply the conjugates in the denominator. .
Simplify the denominator. .

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Simplify: 5 x + 2 .

5 ( x 2 ) x 2

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Simplify: 10 y 3 .

10 ( y + 3 ) y 3

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Simplify: x + 7 x 7 .

Solution

.
Multiply the numerator and denominator by the conjugate of the denominator. .
Multiply the conjugates in the denominator. .
Simplify the denominator. .
We do not square the numerator. In factored form, we can see there are no common factors to remove from the numerator and denominator.

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Simplify: p + 2 p 2 .

( p + 2 ) p 2 2

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Simplify: q 10 q + 10 .

( q 10 ) q 10 2

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Access this online resource for additional instruction and practice with dividing and rationalizing.

Key concepts

  • Quotient Property of Square Roots
    • If a , b are non-negative real numbers and b 0 , then
      a b = a b and a b = a b
  • Simplified Square Roots
    A square root is considered simplified if there are
    • no perfect square factors in the radicand
    • no fractions in the radicand
    • no square roots in the denominator of a fraction

Practice makes perfect

Divide Square Roots

In the following exercises, simplify.

8 x 6 2 x 2 200 m 5 98 m

2 x 2 10 m 2 7

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10 y 3 5 y 108 n 7 243 n 3

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108 p 5 q 2 34 p 3 q 6

6 p q 2

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320 m n 5 45 m 7 n 3

8 n 3 m 3

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Rationalize a One-Term Denominator

In the following exercises, simplify and rationalize the denominator.

Rationalize a Two-Term Denominator

In the following exercises, simplify by rationalizing the denominator.

3 3 + 11 8 1 5

3 ( 3 11 ) −2 −2 ( 1 + 5 )

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4 4 + 7 7 2 6

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5 5 + 6 6 3 7

5 ( 5 6 ) 19 3 ( 3 + 7 )

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6 6 + 5 5 4 11

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3 m 5

3 ( m + 5 ) m 5

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2 x 6

2 ( x + 3 ) x 6

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r + 5 r 5

( r + 5 ) r 5 2

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150 x 2 y 6 6 x 4 y 2

5 y 2 x

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3 5 + 5

3 ( 5 5 ) 20

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2 x 3

2 ( x + 3 ) x 3

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x + 8 x 8

( x + 2 2 ) x 8 2

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Everyday math

A supply kit is dropped from an airplane flying at an altitude of 250 feet. Simplify 250 16 to determine how many seconds it takes for the supply kit to reach the ground.

5 10 4 seconds

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A flare is dropped into the ocean from an airplane flying at an altitude of 1,200 feet. Simplify 1200 16 to determine how many seconds it takes for the flare to reach the ocean.

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Writing exercises


Simplify 27 3 and explain all your steps.
Simplify 27 5 and explain all your steps.
Why are the two methods of simplifying square roots different?

Answers will vary.

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  1. Approximate 1 2 by dividing 1 1.414 using long division without a calculator.
  2. Rationalizing the denominator of 1 2 gives 2 2 . Approximate 2 2 by dividing 1.414 2 using long division without a calculator.
  3. Do you agree that rationalizing the denominator makes calculations easier? Why or why not?
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Self check

After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.

This table has four columns and four rows. The columns are labeled, “I can…,” “confidently.,” “with some help.,” and “no – I don’t get it!” The rows under the column “I can…” read, “divide square roots,” “rationalize a one term denominator.,” and “rationalize a two term denominator.” All the other rows under the columns are empty.

After looking at the checklist, do you think you are well-prepared for the next section? Why or why not?

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Source:  OpenStax, Elementary algebra. OpenStax CNX. Jan 18, 2017 Download for free at http://cnx.org/content/col12116/1.2
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