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Sometimes the coefficient can be factored from all three terms of the trinomial. This will be our strategy in the next example.

Solve 3 x 2 12 x 15 = 0 by completing the square.

Solution

To complete the square, we need the coefficient of x 2 to be one. If we factor out the coefficient of x 2 as a common factor, we can continue with solving the equation by completing the square.

.
Factor out the greatest common factor. .
Divide both sides by 3 to isolate the trinomial. .
Simplify. .
Subtract 5 to get the constant terms on the right. .
Take half of 4 and square it. ( 1 2 ( 4 ) ) 2 = 4 .
Add 4 to both sides. .
Factor the perfect square trinomial as a binomial square. .
Use the Square Root Property. .
Solve for x. .
Rewrite to show 2 solutions. .
Simplify. .
Check.
.

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Solve 2 m 2 + 16 m 8 = 0 by completing the square.

m = −4 ± 2 5

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Solve 4 n 2 24 n 56 = 8 by completing the square.

n = −2 , 8

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To complete the square, the leading coefficient must be one. When the leading coefficient is not a factor of all the terms, we will divide both sides of the equation by the leading coefficient. This will give us a fraction for the second coefficient. We have already seen how to complete the square with fractions in this section.

Solve 2 x 2 3 x = 20 by completing the square.

Solution

Again, our first step will be to make the coefficient of x 2 be one. By dividing both sides of the equation by the coefficient of x 2 , we can then continue with solving the equation by completing the square.

.
Divide both sides by 2 to get the coefficient of x 2 to be 1. .
Simplify. .
Take half of 3 2 and square it. ( 1 2 ( 3 2 ) ) 2 = 9 16 .
Add 9 16 to both sides. .
Factor the perfect square trinomial as a binomial square. .
Add the fractions on the right side. .
Use the Square Root Property. .
Simplify the radical. .
Solve for x. .
Rewrite to show 2 solutions. .
Simplify. .
Check. We leave the check for you.
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Solve 3 r 2 2 r = 21 by completing the square.

r = 7 3 , r = 3

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Solve 4 t 2 + 2 t = 20 by completing the square.

t = 5 2 , t = 2

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Solve 3 x 2 + 2 x = 4 by completing the square.

Solution

Again, our first step will be to make the coefficient of x 2 be one. By dividing both sides of the equation by the coefficient of x 2 , we can then continue with solving the equation by completing the square.

.
Divide both sides by 3 to make the coefficient of x 2 equal 1. .
Simplify. .
Take half of 2 3 and square it. ( 1 2 2 3 ) 2 = 1 9 .
Add 1 9 to both sides. .
Factor the perfect square trinomial as a binomial square. .
Use the Square Root Property. .
Simplify the radical. .
Solve for x . .
Rewrite to show 2 solutions. .
Check. We leave the check for you.
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Solve 4 x 2 + 3 x = 12 by completing the square.

x = 3 8 ± 201 8

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Solve 5 y 2 + 3 y = 10 by completing the square.

y = 3 10 ± 209 10

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Access these online resources for additional instruction and practice with solving quadratic equations by completing the square:

Key concepts

  • Binomial Squares Pattern If a , b are real numbers,
    ( a + b ) 2 = a 2 + 2 a b + b 2

    ( a b ) 2 = a 2 2 a b + b 2
  • Complete a Square
    To complete the square of x 2 + b x :
    1. Identify b , the coefficient of x .
    2. Find ( 1 2 b ) 2 , the number to complete the square.
    3. Add the ( 1 2 b ) 2 to x 2 + b x .

Practice makes perfect

Complete the Square of a Binomial Expression

In the following exercises, complete the square to make a perfect square trinomial. Then, write the result as a binomial squared.

m 2 24 m

( m 12 ) 2

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p 2 22 p

( p 11 ) 2

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x 2 9 x

( x 9 2 ) 2

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p 2 1 3 p

( p 1 6 ) 2

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Solve Quadratic Equations of the Form x 2 + b x + c = 0 by Completing the Square

In the following exercises, solve by completing the square.

v 2 + 6 v = 40

v = −10 , v = 4

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u 2 + 2 u = 3

u = −3 , u = 1

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c 2 12 c = 13

c = −1 , c = 13

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x 2 20 x = 21

x = −1 , x = 21

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m 2 + 4 m = −44

no real solution

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r 2 + 6 r = −11

no real solution

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a 2 10 a = −5

a = 5 ± 2 5

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u 2 14 u + 12 = −1

u = 1 , u = 13

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v 2 = 9 v + 2

v = 9 2 ± 89 2

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( x + 6 ) ( x 2 ) = 9

x = −7 , x = 3

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( y + 9 ) ( y + 7 ) = 79

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Solve Quadratic Equations of the Form a x 2 + b x + c = 0 by Completing the Square

In the following exercises, solve by completing the square.

3 m 2 + 30 m 27 = 6

m = −11 , m = 1

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2 c 2 + c = 6

c = −2 , c = 3 2

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2 p 2 + 7 p = 14

p = 7 4 ± 161 4

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Everyday math

Rafi is designing a rectangular playground to have an area of 320 square feet. He wants one side of the playground to be four feet longer than the other side. Solve the equation p 2 + 4 p = 320 for p , the length of one side of the playground. What is the length of the other side?

16 feet, 20 feet

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Yvette wants to put a square swimming pool in the corner of her backyard. She will have a 3 foot deck on the south side of the pool and a 9 foot deck on the west side of the pool. She has a total area of 1080 square feet for the pool and two decks. Solve the equation ( s + 3 ) ( s + 9 ) = 1080 for s , the length of a side of the pool.

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Writing exercises

Solve the equation x 2 + 10 x = −25 by using the Square Root Property and by completing the square. Which method do you prefer? Why?

−5 −5 Answers will vary.

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Solve the equation y 2 + 8 y = 48 by completing the square and explain all your steps.

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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 rows and four columns. The first row is a header row and it labels each column. The first column is labeled “I can ...”, the second “Confidently”, the third “With some help” and the last “No–I don’t get it”. In the “I can...” column the next row reads “complete the square of a binomial expression.” The next row reads “solve quadratic equations of the form x squared plus b x plus c equals zero by completing the square.” and the last row reads “solve quadratic equations of the form a x squared plus b x plus c equals zero by completing the square.” The remaining columns are blank.

After reviewing this checklist, what will you do to become confident for all objectives?

Practice Key Terms 1

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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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