Review Objectives
- Factor out the greatest common factor of a polynomial
- Factor by grouping
- Factor trinomials (when leading coefficient is one and when leading coefficient is not one)
- Factor perfect square trinomials and difference of squares
Click on the following links to move to different types of factoring on this page.
Factor out the GCF
Factoring 4-term Polynomials by Grouping
Factoring Trinomials with Leading Coefficient of 1
Factoring trinomials with Leading Coefficient not 1
Factoring Special Cases
Factoring Basics
Factors are the building blocks of multiplication. They are the integers that you can multiply together to produce another integer: [latex]2[/latex] and [latex]10[/latex] are factors of [latex]20[/latex], as are [latex]4, 5, 1, 20[/latex]. To factor an integer is to rewrite it as a product. [latex]20=4\cdot{5}[/latex] or [latex]20=1\cdot{20}[/latex]. In algebra, we use the word factor as both a noun – something being multiplied – and as a verb – the action of rewriting a sum or difference as a product. Factoring is very helpful in simplifying expressions and solving equations involving polynomials.
The greatest common factor (GCF) of two integers is the largest common factor to each integer. For instance, [latex]4[/latex] is the GCF of [latex]16[/latex] and [latex]20[/latex] because it is the largest integer that divides evenly into both [latex]16[/latex] and [latex]20[/latex]. The GCF of polynomials works the same way: [latex]4x[/latex] is the GCF of [latex]16x[/latex] and [latex]20{x}^{2}[/latex] because it is the largest polynomial that divides evenly into both [latex]16x[/latex] and [latex]20{x}^{2}[/latex].
When factoring a polynomial expression, our first step should be to check for a GCF. Look for the GCF of the coefficients, and then look for the GCF of the variables.
Greatest Common Factor
The greatest common factor (GCF) of a group of given polynomials is the largest integer and highest degree of each variable that will divide evenly into each term of the polynomial.
Example
Find the greatest common factor of [latex]25b^{3}[/latex] and [latex]10b^{2}[/latex].
In the example above, the monomials have the factors [latex]5[/latex], [latex] b [/latex], and [latex] b [/latex] in common, which means their greatest common factor is [latex]5\cdot{b}\cdot{b}[/latex], or simply [latex]5b^{2}[/latex].
The video that follows gives an example of finding the greatest common factor of two monomials with only one variable.
Sometimes you may encounter a polynomial with more than one variable, so it is important to check whether both variables are part of the GCF. In the next example, we find the GCF of two terms which both contain two variables.
Example
Find the greatest common factor of [latex]81c^{3}d[/latex] and [latex]45c^{2}d^{2}[/latex].
The video that follows shows another example of finding the greatest common factor of two monomials with more than one variable.
You might be picking up on a useful shortcut for the variables. Notice that (assuming the variable is in common) we always select the smaller power of the variable.
Factoring out the Greatest Common Factor (GCF)
Now that you have practiced identifying the GCF of terms with one and two variables, we can apply this idea to factoring the GCF out of a polynomial. Notice that the instructions are now “Factor” instead of “Find the greatest common factor.”
To factor a polynomial, first identify the greatest common factor of the terms. You can then use the distributive property to rewrite the polynomial in factored form.
Distributive Property Forward and Backward
Forward: Product of a number and a sum: [latex]a\left(b+c\right)=a\cdot{b}+a\cdot{c}[/latex]. You can say that “[latex]a[/latex] is being distributed over [latex]b+c[/latex].”
Backward: Sum of the products: [latex]a\cdot{b}+a\cdot{c}=a\left(b+c\right)[/latex]. Here you can say that “[latex]a[/latex] is being factored out.”
We first learned that we could distribute a factor over a sum or difference, now we are learning that we can “undo” the distributive property with factoring.
Example
Factor [latex]25b^{3}+10b^{2}[/latex].
The factored form of the polynomial [latex]25b^{3}+10b^{2}[/latex] is [latex]5b^{2}\left(5b+2\right)[/latex]. You can check this by doing the multiplication. [latex]5b^{2}\left(5b+2\right)=25b^{3}+10b^{2}[/latex].
Note that if you do not factor the greatest common factor at first, you can continue factoring, rather than start all over.
For example:
[latex]\begin{array}{l}25b^{3}+10b^{2}=5\left(5b^{3}+2b^{2}\right)\,\,\,\,\,\,\,\,\,\,\,\text{Factor out }5\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,=5b^{2}\left(5b+2\right) \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\text{Factor out }b^{2}\end{array}[/latex]
Notice that you arrive at the same simplified form whether you factor out the GCF immediately or if you pull out factors individually.
In the following video, we show two more examples of how to find and factor the GCF from binomials.
How To: Given a polynomial expression, factor out the greatest common factor
- Identify the GCF of the coefficients.
- Identify the GCF of the variables.
- Write together to find the GCF of the expression.
- Determine what the GCF needs to be multiplied by to obtain each term in the expression.
- Write the factored expression as the product of the GCF and the sum of the terms we need to multiply by.
We will show a few examples of finding the GCF of a polynomial with several terms and sometimes with two variables. No matter how many terms the polynomial has, you can use the same technique described above to factor out its GCF.
Example
Factor out the GCF. [latex]6{x}^{3}{y}^{3}+45{x}^{2}{y}^{2}+21xy[/latex]
In the following video, you will see two more example of how to find and factor out the greatest common factor of a polynomial.
In the next example, the leading coefficient is negative. In this case, it is convention to include the negative with the GCF. In addition, we will see that this can assist us in other ways as well.
Example
Factor out the GCF. [latex] -24x^8+32x^5 [/latex]
Another interesting situation we may encounter is where the GCF could even include an entire expression containing multiple terms, as shown in the next example.
Example
Factor out the GCF. [latex] 3x^2(2x-5)+7(2x-5) [/latex]
Factor by Grouping
Factoring by grouping is a technique that allows us to factor a polynomial whose terms don’t all share a GCF. In the following example, we will introduce you to the technique. Remember, some of the main reasons we want to be able to factor well is because it will help solve polynomial equations and graph quadratic functions more easily.
Example
Factor completely. [latex] a^2+3a+5a+15 [/latex]
Notice that with this process, we have been able to factor a four-term polynomial into the product of two binomials.
This process is called the grouping technique . Broken down into individual steps, here’s how we apply this technique to a four-term polynomial (you can also follow this process in the example below).
How To: Given a 4-term polynomial expression, factor by grouping
- Group the terms with common factors. Sometimes it will be necessary to rearrange the terms.
- Find the greatest common factor of each pair and factor it out.
- Look for the common binomial between the factored terms.
- If the two terms share a common binomial factor, factor the common binomial out of the groups.
It is worth noting that grouping can be used for polynomials with more than four terms, but the steps above and our examples will focus on four-term polynomials. Let’s try factoring a few more. Note how there is a now a coefficient in front of the [latex] x^2 [/latex] term. We will just consider this another factor when we are finding the GCF.
Example
Factor completely. [latex] 2x^2+4x+5x+10 [/latex]
The following video provides another example of factor by grouping.
Example
Factor completely. [latex] 3x^2 +3x -2x -2 [/latex]
In the following video we present another example of factor by grouping.
ExAMPLE
Factor completely. [latex] 4x^3+12x^2+x+3 [/latex]
Factoring Trinomials with Leading Coefficient of 1
Trinomials are polynomials with three terms. We are going to show you a method for factoring a trinomial whose leading coefficient is [latex]1[/latex]. Although we should always begin by looking for a GCF, factoring out the GCF is not the only way that trinomials can be factored. The trinomial [latex]{x}^{2}+5x+6[/latex] has a GCF of [latex]1[/latex], but it can be written as the product of the binomial factors.
The last example in the previous section asked us to graph [latex] g(x)=x^2+5x+6 [/latex]. It appeared that it would be easier to find the [latex] x [/latex]-intercept(s), vertex, and axis of symmetry if we could change the function to intercept form. How do we change this function from general form [latex] f(x)=ax^2+bx+c [/latex] to intercept (factored) form [latex] f(x)=(x-p)(x-q) [/latex]? We need to learn how to factor trinomials.
We will start by factoring the polynomial [latex] 1x^2+5x+6 [/latex].
Let’s identify the values for [latex] a [/latex], [latex] b [/latex], and [latex] c [/latex]. Looking at the given polynomial, [latex] a=1 [/latex], [latex] b=5 [/latex], and [latex] c=6 [/latex].
We need to find two integers [latex] m [/latex] and [latex] n [/latex] whose product is [latex] c [/latex] and whose sum is [latex] b [/latex]. In other words, let’s think of two integers that multiply to be [latex] 6 [/latex] and add to be [latex] 5 [/latex]. The factors of 6 are
Factors of [latex]6[/latex] | Sum of 5 |
---|---|
[latex]1,6[/latex] | [latex]7[/latex] |
[latex]-1,-6[/latex] | [latex]-7[/latex] |
[latex]-2,-3[/latex] | [latex]-5[/latex] |
[latex]2,3[/latex] | [latex]5[/latex] |
Which pair of factors add to be [latex] 5 [/latex]? The only way to get a sum of 5 is by adding the [latex] 2 [/latex] and [latex] 3 [/latex] ([latex] 2+3=5 [/latex]). We have identified [latex] m [/latex] as [latex] 2 [/latex] and [latex] n [/latex] as [latex] 3 [/latex].
[latex] \begin{align} ax^2+bx+c &= (x+m)(x+n)\\ &= (x+2)(x+3) \end{align} [/latex]
NOTE: The answer could also be written as [latex] (x+3)(x+2) [/latex]. We can change the order of the factors since multiplication is commutative.
To check if it is factored correctly, multiply the two binomials:
[latex]\begin{align}\left(x+2\right)\left(x+3\right) &= x^2+3x+2x+6\\ &= x^2+5x+6 \end{align}[/latex]
Factoring a Trinomial with Leading Coefficient 1
In general, for a trinomial of the form [latex]{x}^{2}+bx+c[/latex], you can factor a trinomial with leading coefficient [latex]1[/latex] by finding two integers, [latex]m[/latex] and [latex]n[/latex], whose product is [latex] c [/latex] and whose sum is [latex] b [/latex].
Let us put this idea to practice with the following example.
Example
Factor completely. [latex]{x}^{2}+2x - 15[/latex]
In the following video, we present two more examples of factoring a trinomial with a leading coefficient of [latex] 1 [/latex].
To summarize our process, consider the following steps:
How To: Given a trinomial in the form [latex]{x}^{2}+bx+c[/latex], factor it
- List factors of [latex]c[/latex].
- Find [latex]m[/latex] and [latex]n[/latex], a pair of factors with a product of [latex]c[/latex] and with a sum of [latex]b[/latex].
- Write the factored expression as [latex]\left(x+m\right)\left(x+n\right)[/latex].
In our next example, we show that when [latex] c [/latex] is negative, either [latex] m [/latex] or [latex] n [/latex] will be negative.
Example
Factor completely. [latex]x^2+x-12[/latex]
Example
Factor completely. [latex]{x}^{2}-7x+6[/latex]
In the last example, the [latex] b [/latex] term was negative and the [latex] c [/latex] term was positive. This will always mean that if it can be factored, [latex] m [/latex] and [latex] n [/latex] will both be negative.
Factoring trinomials with Leading Coefficient not 1
Trinomials with leading coefficients other than 1 are slightly more complicated to factor. For these trinomials, we can factor by grouping by separating the [latex] x [/latex] term into the sum of two terms, factoring each portion of the expression separately, and then factoring out the GCF of the entire expression. The trinomial [latex]2{x}^{2}+5x+3[/latex] can be factored as [latex]\left(2x+3\right)\left(x+1\right)[/latex] using this process. We begin by rewriting the original expression as a 4-term polynomial [latex]2{x}^{2}+2x+3x+3[/latex]. The process to do this is explained in the next example. We will use factor by grouping to complete the factorization of the 4-term polynomial resulting in [latex]2x\left(x+1\right)+3\left(x+1\right)[/latex]. We then pull out the GCF of [latex]\left(x+1\right)[/latex] to find the factored expression.
AC-method of Factoring
To factor a trinomial of the form [latex]a{x}^{2}+bx+c[/latex] by using the AC-Method, we find two integers with a product of [latex]a\cdot c[/latex] and a sum of [latex]b[/latex]. We use these integers to separate the [latex]x[/latex] term into the sum of two terms (making it a 4-term polynomial). Then factor the polynomial by grouping.
Example
Factor [latex]5{x}^{2}+7x - 6[/latex] by grouping.
How To: AC-Method – Given a trinomial in the form [latex]a{x}^{2}+bx+c[/latex],
- Factor out the GCF from all terms, if there is a GCF.
- List factors of [latex]a\cdot c[/latex].
- Find [latex]m[/latex] and [latex]n[/latex], a pair of factors of [latex]a\cdot c[/latex] with a sum of [latex]b[/latex].
- Rewrite the original expression as a 4-term polynomial.
- Factor out the GCF of first two terms.
- Factor out the GCF of last two terms.
- Factor out the GCF of the expression.
Example
Factor completely. [latex] 6x^2+11x+4 [/latex]
ExAMPLE
Factor completely. [latex] 10x^2-7x-6 [/latex]
Example
Factor completely. [latex] 5x^2-x+2 [/latex]
We have a trinomial with [latex]a=5,b=-1[/latex], and [latex]c=-2[/latex]. First, determine [latex]a\cdot c=10[/latex]. We need to find two integers, [latex] m [/latex] and [latex] n [/latex], with a product of [latex]10[/latex] and a sum of [latex]-1[/latex].
Factors of [latex]10[/latex] | Sum of Factors |
---|---|
[latex]1,10[/latex] | [latex]11[/latex] |
[latex]-1,-10[/latex] | [latex]-11[/latex] |
[latex]2,5[/latex] | [latex]7[/latex] |
[latex]-2,-5/latex] | [latex]-7[/latex] |
We have exhausted all the possible integers that multiply to be [latex] 10 [/latex] and none of the combinations of factors will add to be [latex] -1 [/latex]. Therefore, the polynomial is not factorable and is said to be prime.
Answer: Not Factorable or Prime
The following video has another example about factoring trinomials when the leading coefficient is not 1.
In some situations, [latex] a [/latex] is negative, as in [latex] -4h^2+11h+3 [/latex]. It often makes sense to factor out [latex] -1 [/latex] as the first step, making the polynomial easier to factor.
Example
Factor completely. [latex] -4h^2+11h+3 [/latex]
Example
Factor completely. [latex] 16x^4-12x^3-10x^2 [/latex]
Factoring Special Cases
Factoring a Perfect Square Trinomial
What is a perfect square? A perfect square is the product of an integer multiplied by itself. For example, [latex] 4 \cdot 4 = 16 [/latex] So, [latex] 16 [/latex] is a perfect square. Other examples of perfect squares are, [latex] 1, 4, 9, 16, 25, 36, 49, 64, 81 [/latex], and [latex] 100 [/latex]. A perfect square trinomial is a trinomial that can be written in factored form as a binomial squared. Recall that when a binomial is squared, the result is the square of the first term added to twice the product of the two terms and the square of the last term.
Perfect Square Trinomials
A perfect square trinomial can be written, in factored form, as a binomial squared:
Example
Factor completely. [latex]25{x}^{2}+20x+4[/latex]
How To: Given a perfect square trinomial, factor it into the square of a binomial
- Confirm that the first and last term are perfect squares.
- Confirm that the middle term is twice the product of [latex]a\cdot b[/latex].
- Write the factored form as [latex]{\left(a+b\right)}^{2}[/latex] or [latex] {(a-b)}^{2} [/latex].
In the following video, we provide another short description of perfect square trinomials and shows how to factor them using a formula.
Factoring a Difference of Squares
A difference of squares can be factored into two binomials but has only two terms. Let’s start from the product of two special binomials to see the pattern.
Consider the product of the following two binomials, which are identical except for the operation: [latex] (x-2)(x+2) [/latex]. If we multiply them together, the middle term zeros out.
Multiply. [latex] (x-2)(x+2) [/latex]
[latex] \begin{align} &= x^2-2x+2x-2^2\\ &= x^2-2^2\\ &= x^2-4 \end{align} [/latex]
The polynomial [latex] x^2-4 [/latex] is called a difference of squares because each term is a perfect square and the operation connecting them is subtraction. A difference of squares will always factor in the following way:
[latex]\\[/latex]
We can use this equation to factor any differences of squares.
Differences of Squares
A difference of squares can be rewritten as two factors containing the same terms but opposite signs.
How To: Given a difference of squares, factor it into binomials
- Confirm that the first and last term are perfect squares.
- Determine [latex] a [/latex] and [latex] b [/latex].
- Write the factored form as [latex]\left(a+b\right)\left(a-b\right)[/latex].
Example
Factor completely. [latex]9{x}^{2}-25[/latex]
ExAMPLE
Factor completely. [latex] 18x^2-2x [/latex]
Summary
In all factoring problems, we first look for a common factor that can be factored out of all terms. After achieving this (if possible), we have now learned special factoring techniques in this section.
If a trinomial is of the form of a perfect square trinomial, it factors as