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In perfect square trinomials with leading coefficient there is a relationship between the coefficient of the term and the constant.
Considering this relationship, quadratic expressions in the form 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
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.
Discussion
Completing the Square
In a perfect square trinomial, there is a relationship between the coefficient of the term and the constant term — the constant term is equal to the square of half the coefficient of the term.
This relationship can be used to form a perfect square trinomial by adding a constant to any expression in the form
The process of finding the constant can be visualized by using algebraic tiles. Consider the following expression.
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 Term
expand_more For the given expression, the value of is
2
Calculate
expand_more Once the value of is identified, calculate the square of half of the value of
3
Add to the Initial Expression
expand_more Add to the expression to obtain a perfect square trinomial.
In this case, should be added to
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.
This will be applied to the expression
Therefore, a perfect square trinomial is obtained by adding a constant to the initial expression in the form
Pop Quiz
Practicing Finding the Values for Completing the Square
In the following applet, use the method of completing the square to determine the value of that makes the given expression a perfect square trinomial. Round to decimal places if needed.
Discussion
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
Solving a Quadratic Equation by Completing the Square
To solve a quadratic equation by completing the square, write the equation in the form then complete the square of the expression on the left-hand side. Finally, the equation can be solved for by taking square roots of both sides of the equation. To illustrate this, consider the following equation.
To solve the equation by completing the square, these five steps can be followed.
1
Write the Equation in the Form
expand_more The Properties of Equality can be used jointly with inverse operations to rewrite the given equation in the form
If the equation is already in the form this step is skipped.
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 term should be added to each side of the equation.
In the equation found previously, is equal to Therefore, should be added to both sides.
Next, the quotient can be simplified.
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.
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
expand_more Finally, the resulting equations of the previous step need to be solved. These solutions will also be solutions to the original equation.
| Write as two equations | ||
|---|---|---|
| Solve for | ||
Therefore, the solutions of the given equation are and
Example
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 square feet. Additionally, they want the length of the pool to be feet longer than the width.
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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
b Find and interpret the solutions of the quadratic equation by completing the square.
Answer
a
b Solutions: and
Interpretation: The dimensions of the pool cannot be negative. Therefore, the pool will have a width of feet and a lenght of feet.
Interpretation: The dimensions of the pool cannot be negative. Therefore, the pool will have a width of feet and a lenght of 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.
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By substituting the expressions for the length and width of the pool into the formula, the quadratic equation representing the area of the pool can be determined.
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
Example
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 liters and it is expected to be emptied after 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 into the equation for 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 for and solve the equation by completing the square.
The first step is to write the equation in the form
For this equation, the value of is With this information, can be calculated.
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 there will be only one solution for the equation.
This means that there is liter left in the dispenser after minutes.
Example
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 feet, this value will be substituted into the given equation for
The equation must first be written in the form to solve it by completing the square.
Notice that the value of in this equation is To complete the square on the left-hand side, the square of half the value of should be added to both sides.
Therefore, adding 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 feet.
▼
Rewrite
Distr
Distribute
SplitIntoFactors
Split into factors
FactorOut
Factor out
MoveNegNumToFrac
Put minus sign in front of fraction
Closure
Completing the Square to Decipher the Code
In this lesson, quadratic expressions in the form were turned into perfect square trinomials in a process called completing the square. Similarly, quadratic equations were solved by completing the square.
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 units greater than the one before it. Also, the second to last digit is a solution to the following quadratic equation.
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 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
There are two solutions for the equation. However, since the combination lock does not include negative numbers, only makes sense. Therefore, the second to last number of the code is
b Paulina knows that the last number is units greater than the number before it. Therefore, the last number is Now she can fully decipher the code and open the wooden trunk to discover her birthday gifts.
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