7.4 Factor Special Products

Learning Objectives

By the end of this section, you will be able to:

  • Factor perfect square trinomials
  • Factor differences of squares
  • Factor sums and differences of cubes
  • Choose method to factor a polynomial completely

The strategy for factoring we developed in the last section will guide you as you factor most binomials, trinomials, and polynomials with more than three terms. We have seen that some binomials and trinomials result from special products—squaring binomials and multiplying conjugates. If you learn to recognize these kinds of polynomials, you can use the special products patterns to factor them much more quickly.

Factor Perfect Square Trinomials

Some trinomials are perfect squares. They result from multiplying a binomial times itself. You can square a binomial by using FOIL, but using the Binomial Squares pattern you saw in a previous chapter saves you a step. Let’s review the Binomial Squares pattern by squaring a binomial using FOIL.

The first term is the square of the first term of the binomial and the last term is the square of the last. The middle term is twice the product of the two terms of the binomial.

The trinomial 9x2 + 24 +16 is called a perfect square trinomial. It is the square of the binomial 3x+4.

We’ll repeat the Binomial Squares Pattern here to use as a reference in factoring.

When you square a binomial, the product is a perfect square trinomial. In this chapter, you are learning to factor—now, you will start with a perfect square trinomial and factor it into its prime factors.

You could factor this trinomial using the methods described in the last section, since it is of the form ax2 + bx + c. But if you recognize that the first and last terms are squares and the trinomial fits the perfect square trinomials pattern, you will save yourself a lot of work.

Here is the pattern—the reverse of the binomial squares pattern.

To make use of this pattern, you have to recognize that a given trinomial fits it. Check first to see if the leading coefficient is a perfect square, a2. Next check that the last term is a perfect square, b2. Then check the middle term—is it twice the product, 2ab? If everything checks, you can easily write the factors.

The sign of the middle term determines which pattern we will use. When the middle term is negative, we use the pattern a2−2ab+b 2 , which factors to (ab) 2 .

The steps are summarized here.

We’ll work one now where the middle term is negative.

The next example will be a perfect square trinomial with two variables.

Remember the very first step in our Strategy for Factoring Polynomials? It was to ask “is there a greatest common factor?” and, if there was, you factor the GCF before going any further. Perfect square trinomials may have a GCF in all three terms and it should be factored out first. And, sometimes, once the GCF has been factored, you will recognize a perfect square trinomial.

Factor Differences of Squares

The other special product you saw in the previous chapter was the Product of Conjugates pattern. You used this to multiply two binomials that were conjugates. Here’s an example:

Remember, when you multiply conjugate binomials, the middle terms of the product add to 0. All you have left is a binomial, the difference of squares.

Multiplying conjugates is the only way to get a binomial from the product of two binomials.

To factor, we will use the product pattern “in reverse” to factor the difference of squares. A difference of squares factors to a product of conjugates.

Difference of Squares Pattern

If a and b are real numbers,This image shows the difference of two squares formula, a squared – b squared = (a – b)(a + b). Also, the squares are labeled, a squared and b squared. The difference is shown between the two terms. Finally, the factoring (a – b)(a + b) are labeled as conjugates.

Remember, “difference” refers to subtraction. So, to use this pattern you must make sure you have a binomial in which two squares are being subtracted.

It is important to remember that sums of squares do not factor into a product of binomials. There are no binomial factors that multiply together to get a sum of squares. After removing any GCF, the expression a2+b2 is prime!

Don’t forget that 1 is a perfect square. We’ll need to use that fact in the next example.

The binomial in the next example may look “backwards,” but it’s still the difference of squares.

To completely factor the binomial in the next example, we’ll factor a difference of squares twice!

As always, you should look for a common factor first whenever you have an expression to factor. Sometimes a common factor may “disguise” the difference of squares and you won’t recognize the perfect squares until you factor the GCF.

Factor Sums and Differences of Cubes

There is another special pattern for factoring, one that we did not use when we multiplied polynomials. This is the pattern for the sum and difference of cubes. We will write these formulas first and then check them by multiplication.

We’ll check the first pattern and leave the second to you.

The two patterns look very similar, don’t they? But notice the signs in the factors. The sign of the binomial factor matches the sign in the original binomial. And the sign of the middle term of the trinomial factor is the opposite of the sign in the original binomial. If you recognize the pattern of the signs, it may help you memorize the patterns.

The trinomial factor in the sum and difference of cubes pattern cannot be factored.

It can be very helpful if you learn to recognize the cubes of the integers from 1 to 10, just like you have learned to recognize squares. We have listed the cubes of the integers from 1 to 10 in Figure 7.3.

How To

Factor the sum or difference of cubes.

To factor the sum or difference of cubes:

  • Step 1. Does the binomial fit the sum or difference of cubes pattern?
    • Is it a sum or difference?
    • Are the first and last terms perfect cubes?
  • Step 2. Write them as cubes.
  • Step 3. Use either the sum or difference of cubes pattern.
  • Step 4. Simplify inside the parentheses
  • Step 5. Check by multiplying the factors.

Be careful to use the correct signs in the factors of the sum and difference of cubes.

In the next example, we first factor out the GCF. Then we can recognize the sum of cubes.

Media

Access these online resources for additional instruction and practice with factoring special products.

  • Sum of Difference of Cubes
  • Difference of Cubes Factoring

Section 7.4 Exercises

Practice Makes Perfect

Factor Perfect Square Trinomials

In the following exercises, factor.

Factor Sums and Differences of Cubes

In the following exercises, factor.

Mixed Practice

In the following exercises, factor.

Everyday Math

Writing Exercises

Self Check

ⓐ After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.This table has the following statements all to be preceded by “I can…”. The first row is “factor perfect square trinomials”. The second row is “factor differences of squares”. The third row is “factor sums and differences of cubes”. In the columns beside these statements are the headers, “confidently”, “with some help”, and “no-I don’t get it!”.

ⓑ On a scale of 1–10, how would you rate your mastery of this section in light of your responses on the checklist? How can you improve this?