7.5 General Strategy for Factoring Polynomials

Recognize and Use the Appropriate Method to Factor a Polynomial Completely

You have now become acquainted with all the methods of factoring that you will need in this course. (In your next algebra course, more methods will be added to your repertoire.) The figure below summarizes all the factoring methods we have covered. How To: Factor polynomials. outlines a strategy you should use when factoring polynomials.

This figure presents a general strategy for factoring polynomials. First, at the top, there is GCF, which is where factoring starts. Below this, there are three options, binomial, trinomial, and more than three terms. For binomial, there are the difference of two squares, the sum of squares, the sum of cubes, and the difference of cubes. For trinomials, there are two forms, x squared plus bx plus c and ax squared 2 plus b x plus c. There are also the sum and difference of two squares formulas as well as the “a c” method. Finally, for more than three terms, the method is grouping. — **Figure 7.3**

Remember, a polynomial is completely factored if, other than monomials, its factors are prime!

Example 2

Factor completely: $12 x^{2} - 11 x + 2$ .


Is there a GCF?	No.
Is it a binomial, trinomial, or are there more than three terms?	Trinomial.
Are a and c perfect squares?	No, a = 12, not a perfect square.
Use trial and error or the “ac” method. We will use trial and error here.

Check.

$(3 x - 2) (4 x - 1)$

$12 x^{2} - 3 x - 8 x + 2$

$12 x^{2} - 11 x + 2 ✓$

Example 3

Factor completely: $g cubed plus 25 g$ .

Is there a GCF?	Yes, g.	$g cubed plus 25 g$
Factor out the GCF.		$g open parenthesis g squared plus 25 close parenthesis$
In the parentheses, is it a binomial, trinomial, or are there more than three terms?	Binomial.
Is it a sum ? Of squares?	Yes.	Sums of squares are prime.
Check.
Is the expression factored completely?	Yes.
Multiply. $\begin{matrix} g (g^{2} + 25) \\ g^{3} + 25 g ✓ \end{matrix}$

Example 4

Factor completely: $12 y squared minus 75$ .

Is there a GCF?	Yes, 3.	$12 y squared minus 75$
Factor out the GCF.		$3 open parenthesis 4 y squared minus 25 close parenthesis$
In the parentheses, is it a binomial, trinomial, or are there more than three terms?	Binomial.
Is it a sum?	No.
Is it a difference? Of squares or cubes?	Yes, squares.	$3 open parenthesis open parenthesis 2 y close parenthesis squared minus open parenthesis 5 close parenthesis squared close parenthesis$
Write as a product of conjugates.		$3 (2 y - 5) (2 y + 5)$
Check.
Is the expression factored completely?	Yes.
Neither binomial is a difference of squares.
Multiply. $\begin{matrix} 3 (2 y - 5) (2 y + 5) \\ 3 (4 y^{2} - 25) \\ 12 y^{2} - 75 ✓ \end{matrix}$

Example 5

Factor completely: $4 a^{2} - 12 a b + 9 b^{2}$ .

Is there a GCF?	No.
Is it a binomial, trinomial, or are there more terms?
Trinomial with $a not equal to 1$ . But the first term is a perfect square.
Is the last term a perfect square?	Yes.
Does it fit the pattern, $a^{2} - 2 a b + b^{2} ?$	Yes.
Write it as a square.
Check your answer.
Is the expression factored completely?
Yes.
The binomial is not a difference of squares.
Multiply.
$open parenthesis 2 a minus 3 b close parenthesis squared$
${(2 a)}^{2} - 2 \cdot 2 a \cdot 3 b + {(3 b)}^{2}$
$4 a^{2} - 12 a b + 9 b^{2} ✓$

Example 6

Factor completely: $6 y^{2} - 18 y - 60$ .

Is there a GCF?	Yes, 6.	$6 y^{2} - 18 y - 60$
Factor out the GCF.		$6 (y^{2} - 3 y - 10)$
In the parentheses, is it a binomial, trinomial, or are there more terms?	Trinomial with leading coefficient 1.
“Undo” FOIL.	$6 open parenthesis y close parenthesis open parenthesis y close parenthesis$	$6 (y + 2) (y - 5)$
Check your answer.
Is the expression factored completely?		Yes.
Neither binomial is a difference of squares.
Multiply. $\begin{matrix} 6 (y + 2) (y - 5) \\ 6 (y^{2} - 5 y + 2 y - 10) \\ 6 (y^{2} - 3 y - 10) \\ 6 y^{2} - 18 y - 60 ✓ \end{matrix}$

Example 7

Factor completely: $24 x cubed plus 81$ .

Is there a GCF?	Yes, 3.	$24 x cubed plus 81$
Factor it out.		$3 open parenthesis 8 x cubed plus 27 close parenthesis$
In the parentheses, is it a binomial, trinomial, or are there more than three terms?	Binomial.
Is it a sum or difference?	Sum.
Of squares or cubes?	Sum of cubes.
Write it using the sum of cubes pattern.
Is the expression factored completely?	Yes.	$3 (2 x + 3) (4 x^{2} - 6 x + 9)$
Check by multiplying.		We leave the check to you.

Example 8

Factor completely: $2 x^{4} - 32$ .

Is there a GCF?	Yes, 2.	$2 x^{4} - 32$
Factor it out.		$2 (x^{4} - 16)$
In the parentheses, is it a binomial, trinomial, or are there more than three terms?	Binomial.
Is it a sum or difference?	Yes.
Of squares or cubes?	Difference of squares.	$2 open parenthesis open parenthesis x squared close parenthesis squared minus open parenthesis 4 close parenthesis squared close parenthesis$
Write it as a product of conjugates.		$2 (x^{2} - 4) (x^{2} + 4)$
The first binomial is again a difference of squares.		$2 ({(x)}^{2} - {(2)}^{2}) (x^{2} + 4)$
Write it as a product of conjugates.		$2 (x - 2) (x + 2) (x^{2} + 4)$
Is the expression factored completely?	Yes.
None of these binomials is a difference of squares.
Check your answer.
Multiply. $\begin{matrix} 2 (x - 2) (x + 2) (x^{2} + 4) \\ 2 (x^{2} - 4) (x^{2} + 4) \\ 2 (x^{4} - 16) \\ 2 x^{4} - 32 ✓ \end{matrix}$

Example 9

Factor completely: $3 x^{2} + 6 b x - 3 a x - 6 a b$ .

Is there a GCF?	Yes, 3.	$3 x^{2} + 6 b x - 3 a x - 6 a b$
Factor out the GCF.		$3 (x^{2} + 2 b x - a x - 2 a b)$
In the parentheses, is it a binomial, trinomial, or are there more terms?	More than 3 terms.
Use grouping.		$\begin{matrix} 3 [x (x + 2 b) - a (x + 2 b)] \\ 3 (x + 2 b) (x - a) \end{matrix}$
Check your answer.
Is the expression factored completely? Yes.
Multiply. $\begin{matrix} 3 (x + 2 b) (x - a) \\ 3 (x^{2} - a x + 2 b x - 2 a b) \\ 3 x^{2} - 3 a x + 6 b x - 6 a b ✓ \end{matrix}$

Example 10

Factor completely: $10 x^{2} - 34 x - 24$ .

Is there a GCF?	Yes, 2.	$10 x^{2} - 34 x - 24$
Factor out the GCF.		$2 (5 x^{2} - 17 x - 12)$
In the parentheses, is it a binomial, trinomial, or are there more than three terms?	Trinomial with $a not equal to 1.$
Use trial and error or the “ac” method.		$\begin{matrix} 2 (5 x^{2} - 17 x −12) \\ 2 (5 x + 3) (x - 4) \end{matrix}$
Check your answer. Is the expression factored completely? Yes.
Multiply. $\begin{matrix} 2 (5 x + 3) (x - 4) \\ 2 (5 x^{2} - 20 x + 3 x - 12) \\ 2 (5 x^{2} - 17 x - 12) \\ 10 x^{2} - 34 x - 24 ✓ \end{matrix}$

Key Concepts

General Strategy for Factoring Polynomials See Figure 7.3.
How to Factor Polynomials
1. Is there a greatest common factor? Factor it out.
2. Is the polynomial a binomial, trinomial, or are there more than three terms?
  - If it is a binomial:
    Is it a sum?
    - Of squares? Sums of squares do not factor.
    - Of cubes? Use the sum of cubes pattern.
    Is it a difference?
    - Of squares? Factor as the product of conjugates.
    - Of cubes? Use the difference of cubes pattern.
  - If it is a trinomial:
    Is it of the form x2+bx+c? Undo FOIL.
    Is it of the form ax2+bx+c?
    - If ‘a’ and ‘c’ are squares, check if it fits the trinomial square pattern.
    - Use the trial and error or ‘ac’ method.
  - If it has more than three terms:
    Use the grouping method.
3. Check. Is it factored completely? Do the factors multiply back to the original polynomial?

Practice Makes Perfect

Recognize and Use the Appropriate Method to Factor a Polynomial Completely

In the following exercises, factor completely.

$10 x^{4} + 35 x^{3}$

$5 x cubed open parenthesis 2 x plus 7 close parenthesis$

$18 p^{6} + 24 p^{3}$

$y^{2} + 10 y - 39$

$(y - 3) (y + 13)$

$b^{2} - 17 b + 60$

$2 n^{2} + 13 n - 7$

$(2 n - 1) (n + 7)$

$8 x^{2} - 9 x - 3$

$a^{5} + 9 a^{3}$

$a cubed open parenthesis a squared plus 9 close parenthesis$

$75 m cubed plus 12 m$

$121 r squared minus s squared$

$(11 r - s) (11 r + s)$

$49 b squared minus 36 a squared$

$8 m squared minus 32$

$8 (m - 2) (m + 2)$

$36 q squared minus 100$

$25 w^{2} - 60 w + 36$

$open parenthesis 5 w minus 6 close parenthesis squared$

$49 b^{2} - 112 b + 64$

$m^{2} + 14 m n + 49 n^{2}$

$open parenthesis m plus 7 n close parenthesis squared$

$64 x^{2} + 16 x y + y^{2}$

$7 b^{2} + 7 b - 42$

$7 (b + 3) (b - 2)$

$3 n^{2} + 30 n + 72$

$3 x cubed minus 81$

$3 (x - 3) (x^{2} + 3 x + 9)$

$5 t cubed minus 40$

$k^{4} - 16$

$(k - 2) (k + 2) (k^{2} + 4)$

$m^{4} - 81$

$15 p q - 15 p + 12 q - 12$

$3 (5 p + 4) (q - 1)$

$12 a b - 6 a + 10 b - 5$

$4 x^{2} + 40 x + 84$

$4 (x + 3) (x + 7)$

$5 q^{2} - 15 q - 90$

$u^{5} + u^{2}$

$u^{2} (u + 1) (u^{2} - u + 1)$

$5 n cubed plus 320$

$4 c^{2} + 20 c d + 81 d^{2}$

prime

$25 x^{2} + 35 x y + 49 y^{2}$

$10 m^{4} - 6250$

$10 (m - 5) (m + 5) (m^{2} + 25)$

$3 v^{4} - 768$

Everyday Math

Watermelon drop A springtime tradition at the University of California San Diego is the Watermelon Drop, where a watermelon is dropped from the seventh story of Urey Hall.

ⓐ The binomial $−16 t squared plus 80$ gives the height of the watermelon $t$ seconds after it is dropped. Factor the greatest common factor from this binomial.
ⓑ If the watermelon is thrown down with initial velocity 8 feet per second, its height after $t$ seconds is given by the trinomial $−16 t^{2} - 8 t + 80$ . Completely factor this trinomial.

ⓐ $−16 open parenthesis t squared minus 5 close parenthesis$ ⓑ $−8 (2 t + 5) (t - 2)$

Pumpkin drop A fall tradition at the University of California San Diego is the Pumpkin Drop, where a pumpkin is dropped from the eleventh story of Tioga Hall.

ⓐ The binomial $−16 t squared plus 128$ gives the height of the pumpkin t seconds after it is dropped. Factor the greatest common factor from this binomial.
ⓑ If the pumpkin is thrown down with initial velocity 32 feet per second, its height after $t$ seconds is given by the trinomial $−16 t^{2} - 32 t + 128$ . Completely factor this trinomial.

Writing Exercises

The difference of squares $y^{4} - 625$ can be factored as $(y^{2} - 25) (y^{2} + 25)$ . But it is not completely factored. What more must be done to completely factor it?

Answer may vary.

Of all the factoring methods covered in this chapter (GCF, grouping, undo FOIL, ‘ac’ method, special products) which is the easiest for you? Which is the hardest? Explain your answers.

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 row states “recognize and use the appropriate method to factor a polynomial completely”. In the columns beside these statements are the headers, “confidently”, “with some help”, and “no-I don’t get it!”.

ⓑ Overall, after looking at the checklist, do you think you are well-prepared for the next section? Why or why not?