7. Solving Quadratic Equations by Completing the Square
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Chapter 8
7.
Solving Quadratic Equations by Completing the Square
Completing the square is a technique used to solve quadratic equations. The method involves changing the equation into a perfect square trinomial, simplifying the process of finding solutions. Real-world applications include calculating pool dimensions, determining rocket heights, and figuring out how long it takes for a lemonade dispenser to empty. This approach is versatile and can tackle equations that other methods may find challenging. It is especially useful when the equation is not easily factorable or when precise numerical solutions are needed.
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Lesson Settings & Tools
| | 9 Theory slides |
| | 9 Exercises - Grade E - A |
| | Each lesson is meant to take 1-2 classroom sessions |
Solving Quadratic Equations by Completing the Square
Slide of 9
In perfect square trinomials with leading coefficient 1, there is a relationship between the coefficient of the x-term and the constant.
x^2+bx+(b/2 )^2 Considering this relationship, quadratic expressions in the form x^2+bx can be transformed into perfect square trinomials. This lesson will present how these expressions can be transformed into perfect square trinomials and their applications for solving quadratic equations.
Catch-Up and Review
Here are a few recommended readings before getting started with this lesson.
Challenge
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Is It Possible to Rewrite Equations That Are in Standard Form to Find Their Solutions?
Paulina's birthday is this weekend and her parents have hidden her gifts in a trunk. She can have them early if she can open the combination lock on the truck. Her parents gave her the clues to help her find the combination, and she has figured out all but the final two digits.
Paulina knows that the last digit is 2 units greater than the one before it. She also knows that the second to last digit is a solution to the following quadratic equation. x^2+2x-24=0
While considering the equation, Paulina wonders if there is a way to rewrite the given equation so that the variable term appears only once on the left-hand side. If this is possible, what would be its advantages?Discussion
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Completing the Square
In a perfect square trinomial, there is a relationship between the coefficient of the x-term and the constant term — the constant term is equal to the square of half the coefficient of the x-term.
x^2+bx+(b/2)^2
This relationship can be used to form a perfect square trinomial by adding a constant c to any expression in the form x^2+bx.
x^2+bx ⇒ x^2+bx+c
The process of finding the constant c can be visualized by using algebraic tiles. Consider the following expression. x^2+6x
The expression is represented using algebraic tiles. Then, a square is created by rearranging the existing tiles and adding more tiles. The following applet summarizes this process.
This process is called completing the square. To complete the square for an expression algebraically, these steps can be followed.
1
Identify the Coefficient of the x-Term
expand_more For the given expression, the value of b is 6. x^2+ 6x
2
Calculate ( b2)^2
expand_more Once the value of b is identified, calculate the square of half of the value of b.
3
Add ( b2)^2 to the Initial Expression
expand_more Add ( b2)^2 to the expression to obtain a perfect square trinomial. x^2+bx ⇒ x^2+bx+ (b/2)^2 In this case, 9 should be added to x^2+6x. x^2+6x ⇒ x^2+6x+ 9
4
Factor the Perfect Square Trinomial
expand_more The expression obtained in the previous step can be now factored as the square of a binomial. x^2+bx+(b/2)^2=(x+b/2)^2
This will be applied to the expression x^2+6x+9.
Therefore, a perfect square trinomial is obtained by adding a constant c=( b2)^2 to the initial expression in the form x^2+bx.
x^2+6x+9
SplitIntoFactors
Split into factors
x^2+2*3x+9
CommutativePropMult
Commutative Property of Multiplication
x^2+2x*3+9
FacPosPerfectSquare
a^2+2ab+b^2=(a+b)^2
(x+3)^2
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Practicing Finding the Values for Completing the Square
In the following applet, use the method of completing the square to determine the value of c that makes the given expression a perfect square trinomial. Round to 2 decimal places if needed.
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Extending Completing the Square to Quadratic Equations
The most useful application of completing the square is that it can be extended to solve quadratic equations. However, some additional steps need to be taken when using this method to solve equations.
Method
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Solving a Quadratic Equation by Completing the Square
To solve a quadratic equation by completing the square, write the equation in the form x^2+bx=c, then complete the square of the expression on the left-hand side. Finally, the equation can be solved for x by taking square roots of both sides of the equation. To illustrate this, consider the following equation.
2x^2+12x+4=0
To solve the equation by completing the square, these five steps can be followed.
1
Write the Equation in the Form x^2+bx=c
expand_more The Properties of Equality can be used jointly with inverse operations to rewrite the given equation in the form x^2+bx=c.
If the equation is already in the form x^2+bx=c, this step is skipped.
2x^2+12x+4=0
SplitIntoFactors
Split into factors
2x^2+2*6x+2*2=0
FactorOut
Factor out 2
2(x^2+6x+2)=0
DivEqn
.LHS /2.=.RHS /2.
x^2+6x+2=0
SubEqn
LHS-2=RHS-2
x^2+6x=-2
2
Complete the Square on the Left-Hand Side of the Equation
expand_more To complete the square on the left-hand side of the equation, the square of one-half the coefficient of the x-term should be added to each side of the equation. x^2 + bx = c ⇓ x^2 + bx +(b/2)^2 = c +(b/2)^2 In the equation found previously, b is equal to 6. Therefore, ( 62)^2 should be added to both sides. x^2 + 6x = - 2 ⇓ x^2 + 6x +(6/2)^2 = - 2 +(6/2)^2 Next, the quotient can be simplified. x^2+6x+3^2=-2+3^2 The expression on the left-hand side is now a perfect square trinomial.
3
Factor the Perfect Square Trinomial
expand_more Next, factor the perfect square trinomial.
x^2+6x+3^2=-2+3^2
SplitIntoFactors
Split into factors
x^2+2* x * 3+3^2=-2+3^2
FacPosPerfectSquare
a^2+2ab+b^2=(a+b)^2
(x+3)^2=-2+3^2
(x+3)^2=7
4
Take the Square Root of Both Sides of the Equation
expand_more The square root of both sides of the equation can now be taken to remove the exponent.
5
Solve for x
expand_more Finally, the resulting equations of the previous step need to be solved. These solutions will also be solutions to the original equation.
| x+3 = ± sqrt(7) | ||
|---|---|---|
| Write as two equations | x+3 = sqrt(7) | x+3 = - sqrt(7) |
| Solve for x | x =- 3+ sqrt(7) | x = -3- sqrt(7) |
Therefore, the solutions of the given equation are x=- 3+sqrt(7) and x=- 3-sqrt(7).
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Solving Quadratic Equations by Completing the Square
While Paulina thinks about finding the missing digits of the combination lock, her older brother, Vincenzo, and her parents are setting up a rectangular pool for her birthday party. The pool will be in the backyard and will cover an area of 768 square feet. Additionally, they want the length of the pool to be 32 feet longer than the width.
External credits: @kdekiara
Answer the following questions to help Vincenzo and his parents find the dimensions they should use for the pool.
a Write a quadratic equation representing the area of the pool in terms of the width x.
b Find and interpret the solutions of the quadratic equation by completing the square.
Answer
a x^2+32x=768
b Solutions: x= 16 and x=-48
Interpretation: The dimensions of the pool cannot be negative. Therefore, the pool will have a width of 16 feet and a lenght of 48 feet.
Interpretation: The dimensions of the pool cannot be negative. Therefore, the pool will have a width of 16 feet and a lenght of 48 feet.
Hint
a Use the formula for the area of a rectangle.
b The dimensions of the pool cannot be negative.
Solution
a The formula for the area of a rectangle will be used to create the quadratic equation representing the area of the pool.
A=l w In this formula, A represents the area, l the length, and w the width of the rectangle. It is given that the area of the pool must be 768 square feet. A=l w Substitute 768=l w Now, let x represent the width of the pool. Because the length will be 32 feet longer than the width, it can be represented as x+32.
External credits: @kdekiara
b To solve the quadratic equation written in Part A, the method of completing square will be used. Notice that the equation is already in the form x^2+bx=c.
x^2+32x+16^2=768+16^2
SplitIntoFactors
Split into factors
x^2+2* x*16+16^2=768+16^2
FacPosPerfectSquare
a^2+2ab+b^2=(a+b)^2
(x+16)^2=768+16^2
CalcPow
Calculate power
(x+16)^2=768+256
AddTerms
Add terms
(x+16)^2=1024
(x+16)^2=1024
SqrtEqn
sqrt(LHS)=sqrt(RHS)
sqrt((x+16)^2)=sqrt(1024)
sqrt(a^2)=± a
x+16= ± sqrt(1024)
CalcRoot
Calculate root
x+16= ± 32
| x+16= ± 32 | |
|---|---|
| x+16= 32 | x+16= - 32 |
| x=16 | x= - 48 |
The quadratic equation has two solutions, one negative and one positive. Because the dimensions of the pool cannot be negative, the negative solution is not an option for Vincenzo. Therefore, the dimensions of the pool are given by the positive solution. Width:&16 ft^2 Length:&16+32=48ft^2
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Completing the Square to Solve Daily Problems
At Paulina's birthday party, there will be a lemonade dispenser that automatically fills people's glasses. The dispenser has a capacity of 40 liters and it is expected to be emptied after 60 minutes. The dispenser must be refilled when there is only one liter of lemonade left in it in order for the automatic filling function to work.
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Hint
Substitute 1 into the equation for V and solve it by completing the square.
Solution
It is asked to find how long it will take to have only one liter left in the dispenser. To do so, substitute 1 for V and solve the equation by completing the square.
1=t^2-110t+3026 ⇕ t^2-110t+3026= 1
The first step is to write the equation in the form x+bx=c.
For this equation, the value of b is -110. With this information, ( b2)^2 can be calculated.
(- 110/2)^2 ⇔ (- 55)^2 = 55^2
A perfect square trinomial is formed on the left-hand side of the equation by adding this term to both sides of the equation. This trinomial can then be factored.
Now, the square root can be applied to both sides of the equation to find its solutions. However, because the right-hand side of the equation is 0, there will be only one solution for the equation.
This means that there is 1 liter left in the dispenser after 55 minutes.
t^2-110t=-3025
AddEqn
LHS+55^2=RHS+55^2
t^2-110t+55^2=-3025+55^2
SplitIntoFactors
Split into factors
t^2-2* t*55 +55^2=-3025+55^2
FacNegPerfectSquare
a^2-2ab+b^2=(a-b)^2
(t-55)^2=-3025+55^2
(t-55)^2=0
(t-55)^2=0
▼
Solve for t
SqrtEqn
sqrt(LHS)=sqrt(RHS)
sqrt((t-55)^2)=sqrt(0)
CalcRoot
Calculate root
sqrt((t-55)^2)=0
SqrtPowToNumber
sqrt(a^2)=a
t-55 = 0
AddEqn
LHS+55=RHS+55
t=55
Example
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Completing the Square to Identify Quadratic Equations With No Solutions
Dominika and Heichi built a small rocket for Paulina's birthday party. They all excitedly decide to launch it at the end of her birthday party.
Answer
No, see solution.
Hint
The square of a real number cannot be negative.
Solution
To find if the rocket can reach a height of 60 feet, this value will be substituted into the given equation for h. 60=-16t^2+32t+12 ⇕ -16t^2+32t+12= 60
The equation must first be written in the form x^2+bx=c to solve it by completing the square.
Notice that the value of b in this equation is - 2. To complete the square on the left-hand side, the square of half the value of b, ( - 22 )^2, should be added to both sides.
(- 2/2)^2 ⇔ (- 1)^2=1
Therefore, adding 1 to both sides of the equation will produce a perfect square trinomial on the left-hand side that can be factored as the square of a binomial.
Note that the right-hand side is negative. Since there is no real number that has a negative square, the equation has no real solutions. This means that the rocket cannot reach a height of 60 feet.
-16t^2+32t+12=60
SubEqn
LHS-12=RHS-12
-16t^2+32t=48
▼
Rewrite
MultEqn
LHS * (-1)=RHS* (-1)
(-16t^2+32t)(-1)=48(-1)
Distr
Distribute (-1)
(-16t^2)(-1)+(32t)(-1)=48(-1)
MultNegNegTwoPar
(- a)(- b)=a* b
16t^2+(32t)(-1)=48(-1)
MultPosNeg
a(- b)=- a * b
16t^2+(-32t)=-48
AddNeg
a+(- b)=a-b
16t^2-32t=-48
SplitIntoFactors
Split into factors
16* t^2-16*2* t=-16*3
FactorOut
Factor out 16
16(t^2-2t)=-16*3
DivEqn
.LHS /16.=.RHS /16.
t^2-2t=-16*3/16
MoveNegNumToFrac
Put minus sign in front of fraction
t^2-2t=-16*3/16
DenomMultFracToNumber
A * a/A= a
t^2-2t=-3
t^2-2t=-3
AddEqn
LHS+1=RHS+1
t^2-2t+1=-3+1
FacNegPerfectSquare
a^2-2ab+b^2=(a-b)^2
(t-1)^2=-3+1
AddTerms
Add terms
(t-1)^2=-2
Closure
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Completing the Square to Decipher the Code
In this lesson, quadratic expressions in the form x^2+bx were turned into perfect square trinomials in a process called completing the square. Similarly, quadratic equations were solved by completing the square. \begin{gathered} x^2+bx= c \\ \Downarrow \\ \underbrace{x^2+\dfrac{b}x {\color{#0000FF}{+\left(\dfrac{b}{2}\right)^2}}}_{\text{Perfect Square Trinomial}}= c{\color{#0000FF}{+\left(\dfrac{b}{2}\right)^2}} \end{gathered} Considering these methods, the challenge presented at the beginning can now be solved. Recall that Paulina's parents gave her the clues to the combination of the lock on the trunk containing her birthday gifts. Paulina has figured out all but the last two digits of the combination.
She remembers that the last digit 2 units greater than the one before it. Also, the second to last digit is a solution to the following quadratic equation. x^2+2x-24=0 The given equation can be rewritten by completing the square. The advantage of completing the square is that every equation can be solved using this method. Also, once the square is completed, determining the solutions is straightforward. Using the given information, answer the following questions to help Paulina.
a What is the correct number for the second to last digit?
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b What is the last digit of the combination?
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Hint
a Consider the the lock contains only non-negative numbers.
b Add 2 to the second to last number of the code.
Solution
a To find the second to last digit of the code, the given equation will be solved by completing the square. The equation will first be written in the form x+bx=c.
(x+1)^2=25
SqrtEqn
sqrt(LHS)=sqrt(RHS)
sqrt((x+1)^2)=sqrt(25)
sqrt(a^2)=± a
x+1=± sqrt(25)
CalcRoot
Calculate root
x+1=± 5
| x+1=± 5 | |
|---|---|
| x+1= 5 | x+1=- 5 |
| x=4 | x=- 6 |
There are two solutions for the equation. However, since the combination lock does not include negative numbers, only 4 makes sense. Therefore, the second to last number of the code is 4.
b Paulina knows that the last number is 2 units greater than the number before it. Therefore, the last number is 4+ 2=6. Now she can fully decipher the code and open the wooden trunk to discover her birthday gifts.
Solving Quadratic Equations by Completing the Square
Exercise 1.1
Determine the value of c that makes the expression a perfect square trinomial.
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We are asked to find the value of c that makes the given expression a perfect square trinomial. To do this, we will first try to identify the value of b. In a quadratic expression, b is the coefficient of the linear term. x^2+ 36x+c To change the expression x^2+bx into a perfect square trinomial, calculate the square of half of the value of b. In this case, b= 36. Let's calculate it!
Following a similar reasoning as in Part A, let's identify the value of b.
s^2+ 14s+c
We have identified 14 to be the value of b. We will now calculate the square of half of 14.
In a similar fashion, let's find the value of the coefficient of the linear term for the given expression.
r^2+ 42r+c
Let's find the value of ( b2)^2.
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Use the method of completing the square to solve the equation. When entering multiple roots, use the labels to change the number of entries.
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We will find the solutions to the quadratic equation by completing the square. Observing the given equation, we can note that it is already written in the form x^2+bx=c. t^2+ 6t=55 To complete the square, we should add ( b2)^2 to both sides of the equation. In this case, b= 6. Let's first calculate ( b2)^2.
We will now add 9 to both sides of the equation. Then we will factor the resulting perfect square trinomial on the left-hand side.
We will take the square root of both sides of the equation. This will produce two linear equations whose solutions are also solutions of the quadratic equation.
Using the positive and negative signs will give us the solutions to the equation.
| t=-3±8 | |
|---|---|
| t=-3+8 | t=-3-8 |
| t=5 | t=-11 |
We have determined that t=5 and t=-11. These are the solutions to the quadratic equation.
We will begin by writing the given equation in the form x^2+bx=c. q^2-18q+32=0 ⇓ q^2-18q=-32 Note that b=-18. We will calculate ( b2)^2 that will be later added to both sides of the equation.
We will add 81 to both sides of the equation to form a perfect square trinomial on the left-hand side. This trinomial can later be factored as the square of a binomial.
Now, let's apply the square root to both sides of the equation to get two linear equations.
By taking the positive and negative signs, we will get the two solutions to the equation.
| q=9±7 | |
|---|---|
| q=9+7 | q=9-7 |
| q=16 | q=2 |
We have determined that q=16 and q=2 are the solutions of the quadratic equation.
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Solve each equation by completing the square. Round to the nearest tenth if needed.
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We will determine the solutions to the given equation by using the method of completing the square. To do so, we will first factor out 3.
Once the equation is in the form x^2+bx=c, we need to calculate the term that should be added to both sides of the equation to form a perfect square trinomial on its left-hand side. This is given by ( b2)^2. Note that in the resulting equation b= 12.
We will now add 36 to both sides of the equation in the form x^2+bx=c.
By applying the square root to both sides of the equation, we will obtain two linear equations whose solutions are also solutions of the quadratic equation.
We will separate the positive and negative cases and use a calculator to find the solutions to the equation.
| x=-6±sqrt(27) | ||
|---|---|---|
| x=-6+sqrt(27) | x=-6-sqrt(27) | |
| x≈-0.8 | x≈-11.2 | |
Consider the given equation. 5x^2+10x-35=0 We will begin by factoring out 5 and then we will write the equation in the form x^2+bx=c.
Having the equation in the form x^2+bx=c, we will complete the square on the left-hand side. Let's first calculate the missing term ( b2)^2.
This term should be added now to both sides of the equation to produce a perfect square trinomial on the left hand side so that it can be factored as the square of a binomial.
We will now apply the square root to both sides of the equation to get two linear equations.
Finally, let's take the positive and negative cases and use a calculator to obtain the solutions to the equation.
| x=-1±sqrt(8) | ||
|---|---|---|
| x ≈ -1+sqrt(8) | x ≈ -1-sqrt(8) | |
| x≈1.8 | x≈-3.8 | |
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Use the method of completing the square to find the solutions of the equation.
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The given equation is already in the form x^2+bx=c. x^2-5x=-6.25 This means we need to calculate the missing term, ( b2)^2, that should be added to both sides of the equation to obtain a perfect square trinomial on its left-hand side. In the given equation, b= -5.
We will add this term to both sides of the equation and factor the resulting perfect square trinomial.
We will apply the square root to both sides of the equation to find its solutions. However, because the right-hand side of the equation is 0, there will be only one solution for the equation.
Consider the given equation.
x^2-4x=-6
We will calculate the missing term ( b2)^2. In this case, b= -4.
Let's add 4 to both sides of the equation to get a perfect square trinomial.
Next, we should apply the square root to both sides of the equation. However, we can note that the right-hand side is negative. Since there is no real number that has a negative square, the equation has no real solutions.
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Solving Quadratic Equations by Completing the Square
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