Factoring • Factoring Out the GCF • Factoring Difference of Two Squares • Prime Factors • Factoring Completely • Factoring by Grouping
When an algebraic expression is written as the product of two or more quantities, each of these quantities is called a factor of the expression. In practice, we often have an expression and we need to find its factors. Finding these factors, which is essentially reversing the process of multiplication, is called factoring. Factoring is needed in many technical applications that require us to solve higher-degree equations, perform operations on algebraic fractions, or solve certain literal equations.
Since factoring is the reversal of multiplying, it is important that we understand certain types of special products. Each special product, when used in reverse, gives us a method for factoring. The first three sections of this chapter are devoted to discussing different techniques that can be used to factor expressions. In this section, we discuss the first two of these techniques: factoring out the greatest common factor and factoring the difference of squares.
From the distributive law, we know that . When written in reverse, this becomes
Equation (6.1) provides us with a technique of factoring called factoring out the greatest common factor (or GCF, for short). In this case, a is the GCF and it has been factored out of the expression.
To factor out the GCF, we first find the greatest monomial that is common to each term of the expression. This is the GCF. We then use Eq. (6.1) to write the expression as the product of the GCF and the remaining expression. Although we will be discussing several other factoring techniques in this chapter, factoring out the GCF is always the first step in factoring. The following examples illustrate this method.
In factoring we note each term contains a factor of 2:
Here, 2 is the greatest common factor, and is the required factored form of .
[We check the result by multiplication.] In this case,
Since the result of the multiplication gives the original expression, the factored form is correct.
In Example 1, we determined the greatest common factor of 2 by inspection. This is normally the way in which it is found. Once the GCF has been found, the other factor can be determined by dividing the original expression by the GCF.
The next example illustrates the case where the GCF is the same as one of the terms, and special care must be taken to complete the factoring correctly.
Factor: .
The numerical factor 2 and the literal factors a and x are common to each term. Therefore, the greatest common factor of is 2ax. This means that
Note the presence of the 1 in the factored form. When we divide by 2ax, we get
Note that in Example 2, 2ax divided by 2ax is 1 and does not simply cancel out leaving nothing. In cases like this, where the GCF is the same as one of the terms, it is a common error to omit the 1. However, we must include the 1. Without it, when the factored form is multiplied out, we would not obtain the original expression.
Usually, the division shown in Example 2 is done mentally. However, we show it here to emphasize the actual operation that is being performed when we factor out a GCF.
Factor: .
After inspecting each term, we determine that each contains a factor of 3, and . Thus, the greatest common factor is . This means that
When factoring, it is important to factor out the greatest common factor, not just any common factor. Factoring out something other than the GCF will lead to an expression that is not factored completely. To be factored completely, the factors must be prime, meaning they contain no factor other than and itself. The following example illustrates this point.
If we factor the common factor of 2 out of the expression we get .
However, this is not factored completely. The factor is not prime because it may be further factored as .
If we instead factor out the greatest common factor 6x, we get .
This is now factored completely since the factor is prime.
In these examples, note that factoring an expression does not actually change the expression, although it does change the form of the expression. In equating the expression to its factored form, we write an identity.
It is often necessary to use factoring when solving an equation. This is illustrated in the following example.
An equation used in the analysis of FM reception is . Solve for .
The steps in the solution are as follows:
We see that we collected both terms containing on the left so that we could factor and thereby solve for .
From Section 1.8, we know that in order to multiply two binomials, we multiply each term of the first binomial by each term of the second. When we apply this procedure to the special product we get . Since the two terms in the middle add up to zero, this simplifies to the following:
We can use Eq. (6.2) to find this type of special product directly, as shown in the following example:
To find the product which arises from analyzing forces on a beam, we have
An important consequence of Eq. (6.2) is that, when written in reverse, it gives us a method of factoring called factoring the difference of two squares.
Equation (6.3) can be used to factor the difference between two perfect squares as shown in the following examples.
In factoring note that is the square of x and 16 is the square of 4. Therefore,
Because is the square of 2x and 9 is the square of 3, we may factor as
In the same way,
where we note that .
[However, we must be careful to see if the other factor can itself be factored.]
As noted before, the greatest common factor should be factored out first. It is possible, for example, that the other factor is the difference of squares.
This means that complete factoring often requires more than one step. Be sure to include only prime factors in the final result.
In factoring note a common factor of 5 in each term. Therefore, . However, the factor itself is the difference of squares. Therefore, is completely factored as
In factoring note that we have the difference of two squares. Therefore, . However, the factor is also the difference of squares. This means that
In analyzing the energy collected by different circular solar cells, the expression arises. In factoring this expression, we note the factor 5 in each term. Therefore, . However, is the difference of squares. Therefore, factoring the original expression, we have
In Example 9(b), the factor is prime. It is not equal to since .
Terms in an expression can sometimes be grouped and then factored by methods of this section. The following example illustrates this method of factoring by grouping. In the next section, we discuss another type of expression that can be factored by grouping.
Factor: .
We see that there is no common factor to all four terms, but that each of the first two terms contains a factor of 2, and each of the third and fourth terms contains a factor of a. Grouping terms this way and then factoring each group, we have
In Example 2, change the sign to and then factor.
In Example 2, set the given expression equal to B and then solve for a.
In Example 9(a), change the coefficient of the first term from 20 to 5 and then factor.
In Example 10, change both signs to and then factor.
for a
for x
for s
for y
for k
for k
Factor and then explain why it represents a positive even integer if n is a positive integer.
Factor and then explain why it represents a multiple of 6 if n is an integer greater than 1.
(fire science)
(machine design)
(rocket path)
(container design)
(pipeline flow)
(fluid flow)
A square as large as possible is cut from a circular metal plate of radius r. Express in factored form the area of the metal pieces that are left.
A pipe of outside diameter d is inserted into a pipe of inside radius r. Express in factored form the cross-sectional area within the larger pipe that is outside the smaller pipe.
A spherical float has a volume of air within it of radius and the outer radius of the float is . Express in factored form the difference in areas of the outer surface and the inner surface.
The kinetic energy of an object of mass m traveling at velocity v is given by . Suppose a car of mass equipped with a crash-avoidance system automatically applies the brakes to avoid a collision and slows from a velocity of to a velocity of . Find an expression, in factored form, for the difference between the original and final kinetic energy.
for (electricity: ammeter)
for (acoustics)
for B (physics: elasticity)
for q (computer design)
for t (energy conservation)
for k (factor resulting denominator) (radiation)
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