9.8: Factorization of Quadratic Expressions
Suppose your height above sea level in feet when traveling over a hill can be represented by the expression , where is the horizontal distance traveled. If you wanted to factor this expression, could you do it? What would be the steps that you would follow? After completing this Concept, you'll know what to do in order to factor an expression such as this one.
Guidance
In this Concept, we will learn how to factor quadratic polynomials for different values of , and . In the last Concept, we factored common monomials, so you already know how to factor quadratic polynomials where .
Factoring Quadratic Expressions in Standard Form
Quadratic polynomials are polynomials of degree 2. The standard form of a quadratic polynomial is , where , and are real numbers .
Example A
Factor .
Solution: We are looking for an answer that is the product of the two binomials in parentheses: .
To fill in the blanks, we want two numbers and that multiply to 6 and add to 5. A good strategy is to list the possible ways we can multiply two numbers to give us 6 and then see which of these pairs of numbers add to 5. The number six can be written as the product of:
So the answer is .
We can check to see if this is correct by multiplying .
is multiplied by and .
2 is multiplied by and .
Combine the like terms: .
Example B
Factor .
Solution: We are looking for an answer that is the product of the two binomials in parentheses: .
The number 8 can be written as the product of the following numbers.
and Notice that these are two different choices.
And
and This is the correct choice.
The answer is .
Example C
Factor .
Solution: We are looking for an answer that is the product of the two binomials in parentheses: .
In this case, we must take the negative sign into account. The number –15 can be written as the product of the following numbers.
and Notice that these are two different choices.
And also,
and Notice that these are two different choices.
and This is the correct choice.
and
The answer is .
To Summarize:
A quadratic of the form factors as a product of two binomials: .

If
and
are positive then both
and
are positive.
 Example: factors as .

If
is negative and
is positive then both
and
are negative.
 Example: factors as .

If
is negative then either
is positive and
is negative or viceversa.
 Example: factors as .
 Example: factors as .

If
, factor a common factor of –1 from each term in the trinomial and then factor as usual. The answer will have the form
.
 Example: factors as .
Video Review
Guided Practice
Factor .
Solution: First factor the common factor of –1 from each term in the trinomial. Factoring –1 changes the signs of each term in the expression.
We are looking for an answer that is the product of the two binomials in parentheses: .
Now our job is to factor .
The number –6 can be written as the product of the following numbers.
The answer is .
Practice
Sample explanations for some of the practice exercises below are available by viewing the following video. Note that there is not always a match between the number of the practice exercise in the video and the number of the practice exercise listed in the following exercise set. However, the practice exercise is the same in both. CK12 Basic Algebra: Factoring Quadratic Equations (16:30)
Factor the following quadratic polynomials.
Mixed Review
 Evaluate when .

The Nebraska Department of Roads collected the following data regarding mobile phone distractions in traffic crashes by teen drivers.
 Plot the data as a scatter plot.
 Fit a line to this data.
 Predict the number of teenage traffic accidents attributable to cell phones in the year 2012.
Year ( )  Total ( ) 

2002  41 
2003  43 
2004  47 
2005  38 
2006  36 
2007  40 
2008  42 
2009  42 
 Simplify .
 Graph the following on a number line: .
 What is the multiplicative inverse of ?
Quick Quiz
 Name the following polynomial. State its degree and leading coefficient: .
 Simplify .
 A rectangular solid has dimensions by by . Find its volume.
 Simplify .
 Find the solutions to .
 Multiply .
Image Attributions
Description
Learning Objectives
Here you'll learn to determine what the factors of a quadratic expression are.