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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?

Questions & Answers

how did you get 1640
Noor Reply
If auger is pair are the roots of equation x2+5x-3=0
Peter Reply
Wayne and Dennis like to ride the bike path from Riverside Park to the beach. Dennis’s speed is seven miles per hour faster than Wayne’s speed, so it takes Wayne 2 hours to ride to the beach while it takes Dennis 1.5 hours for the ride. Find the speed of both bikers.
MATTHEW Reply
420
Sharon
from theory: distance [miles] = speed [mph] × time [hours] info #1 speed_Dennis × 1.5 = speed_Wayne × 2 => speed_Wayne = 0.75 × speed_Dennis (i) info #2 speed_Dennis = speed_Wayne + 7 [mph] (ii) use (i) in (ii) => [...] speed_Dennis = 28 mph speed_Wayne = 21 mph
George
Let W be Wayne's speed in miles per hour and D be Dennis's speed in miles per hour. We know that W + 7 = D and W * 2 = D * 1.5. Substituting the first equation into the second: W * 2 = (W + 7) * 1.5 W * 2 = W * 1.5 + 7 * 1.5 0.5 * W = 7 * 1.5 W = 7 * 3 or 21 W is 21 D = W + 7 D = 21 + 7 D = 28
Salma
Devon is 32 32​​ years older than his son, Milan. The sum of both their ages is 54 54​. Using the variables d d​ and m m​ to represent the ages of Devon and Milan, respectively, write a system of equations to describe this situation. Enter the equations below, separated by a comma.
Aaron Reply
find product (-6m+6) ( 3m²+4m-3)
SIMRAN Reply
-42m²+60m-18
Salma
what is the solution
bill
how did you arrive at this answer?
bill
-24m+3+3mÁ^2
Susan
i really want to learn
Amira
I only got 42 the rest i don't know how to solve it. Please i need help from anyone to help me improve my solving mathematics please
Amira
Hw did u arrive to this answer.
Aphelele
hi
Bajemah
-6m(3mA²+4m-3)+6(3mA²+4m-3) =-18m²A²-24m²+18m+18mA²+24m-18 Rearrange like items -18m²A²-24m²+42m+18A²-18
Salma
complete the table of valuesfor each given equatio then graph. 1.x+2y=3
Jovelyn Reply
x=3-2y
Salma
y=x+3/2
Salma
Hi
Enock
given that (7x-5):(2+4x)=8:7find the value of x
Nandala
3x-12y=18
Kelvin
please why isn't that the 0is in ten thousand place
Grace Reply
please why is it that the 0is in the place of ten thousand
Grace
Send the example to me here and let me see
Stephen
A meditation garden is in the shape of a right triangle, with one leg 7 feet. The length of the hypotenuse is one more than the length of one of the other legs. Find the lengths of the hypotenuse and the other leg
Marry Reply
how far
Abubakar
cool u
Enock
state in which quadrant or on which axis each of the following angles given measure. in standard position would lie 89°
Abegail Reply
hello
BenJay
hi
Method
I am eliacin, I need your help in maths
Rood
how can I help
Sir
hmm can we speak here?
Amoon
however, may I ask you some questions about Algarba?
Amoon
hi
Enock
what the last part of the problem mean?
Roger
The Jones family took a 15 mile canoe ride down the Indian River in three hours. After lunch, the return trip back up the river took five hours. Find the rate, in mph, of the canoe in still water and the rate of the current.
cameron Reply
Shakir works at a computer store. His weekly pay will be either a fixed amount, $925, or $500 plus 12% of his total sales. How much should his total sales be for his variable pay option to exceed the fixed amount of $925.
mahnoor Reply
I'm guessing, but it's somewhere around $4335.00 I think
Lewis
12% of sales will need to exceed 925 - 500, or 425 to exceed fixed amount option. What amount of sales does that equal? 425 ÷ (12÷100) = 3541.67. So the answer is sales greater than 3541.67. Check: Sales = 3542 Commission 12%=425.04 Pay = 500 + 425.04 = 925.04. 925.04 > 925.00
Munster
difference between rational and irrational numbers
Arundhati Reply
When traveling to Great Britain, Bethany exchanged $602 US dollars into £515 British pounds. How many pounds did she receive for each US dollar?
Jakoiya Reply
how to reduced echelon form
Solomon Reply
Jazmine trained for 3 hours on Saturday. She ran 8 miles and then biked 24 miles. Her biking speed is 4 mph faster than her running speed. What is her running speed?
Zack Reply
d=r×t the equation would be 8/r+24/r+4=3 worked out
Sheirtina
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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