# § 5.4 Factoring Trinomials.

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§ 5.4 Factoring Trinomials

A Strategy for Factoring Polynomials, page 363
If there is a common factor, factor out the GCF or factor out a common factor with a negative coefficient. Determine the number of terms in the polynomial and try factoring as follows: (a) If there are two terms, can the binomial be factored by using one of the following special forms. Difference of two squares: Sum and Difference of two cubes: (b) If there are three terms, If is the trinomial a perfect square trinomial use one of the adjacent forms: If the trinomial is not a perfect square trinomial, If a is equal to 1, use the trial and error If a is > than 1, use the grouping method (c) If there are four or more terms, try factoring by grouping. /////////////////////////////////////////// Blitzer, Intermediate Algebra, 5e – Slide #2 Section 5.6

A Strategy for Factoring T
Factoring Trinomials, p 340 A Strategy for Factoring T 1) Enter x as the first term of each factor. 2) List pairs of factors of the constant c. 3) Try various combinations of these factors. Select the combination in which the sum of the Outside and Inside products is equal to bx. 4) Check your work by multiplying the factors using the FOIL method. You should obtain the original trinomial. If none of the possible combinations yield an Outside product and an Inside product who sum is equal to bx, the trinomial cannot be factored using integers and is called prime over the set of integers. I Sum of O + I O Blitzer, Intermediate Algebra, 5e – Slide #3 Section 5.4

Factoring Trinomials Factor: EXAMPLE SOLUTION
1) Enter x as the first term of each factor. To find the second term of each factor, we must find two integers whose product is 24 and whose sum is 11. 2) List pairs of factors of the constant, 24. Factors of 24 24, 1 -24, -1 12, 2 -12, -2 8, 3 -8, -3 6, 4 -6, -4 OR Factors of 24 without repeating the negative numbers Sum 1 24 25 2 12 14 3 8 11 4 6 10 Blitzer, Intermediate Algebra, 5e – Slide #4 Section 5.4

Factoring Trinomials CONTINUED 3) Try various combinations of these factors. The correct factorization of is the one in which the sum of the Outside and Inside products is equal to 11x. Here is a list of the possible factorizations. Possible Factorizations of Sum of Outside and Inside Products (Should Equal 11x) (x + 24)(x + 1) x + 24x = 25x (x - 24)(x - 1) -x - 24x = -25x (x + 12)(x + 2) 2x + 12x = 14x (x - 12)(x - 2) -2x - 12x = -14x (x + 8)(x + 3) 3x + 8x = 11x (x - 8)(x - 3) -3x - 8x = -11x (x + 6)(x + 4) 4x + 6x = 10x (x - 6)(x - 4) -4x - 6x = 10x This is the required middle term. Sum 1 24 25 2 12 14 3 8 11 4 6 10 Blitzer, Intermediate Algebra, 5e – Slide #5 Section 5.4

Factoring Trinomials Thus,
CONTINUED Thus, Check this result by multiplying the right side using the FOIL method. You should obtain the original trinomial. Because of the commutative property, the factorization can also be expressed as Blitzer, Intermediate Algebra, 5e – Slide #6 Section 5.4

Factoring Trinomials Factor: Enter a as the first term of each factor.
EXAMPLE Factor: SOLUTION Enter a as the first term of each factor. Find two integers whose product is -14 and whose sum is 5. Difference 1 14 13 2 7 5 Thus, 7 minus 2 is 5 This is the desired difference. Blitzer, Intermediate Algebra, 5e – Slide #7 Section 5.4

Factoring Trinomials Check Point 1 page 342 Factor:
Find two integers whose product is 8 and whose sum is 6. Sum 1 8 9 2 4 6 Check Point 2 page 342 Sum 1 20 21 2 10 12 4 5 9 Factor: Find two integers whose product is 20 and whose sum is 9. Blitzer, Intermediate Algebra, 5e – Slide #8 Section 5.4

Factoring Trinomials Check Point 3 page 343 Factor:
Find two integers whose product is 66 and whose sum is 19. Difference 1 66 65 2 33 29 3 22 19 6 11 5 Number 22 page 350 Sum 1 8 9 2 4 6 Factor: No solution. PRIME Find two integers whose product is 8 and whose sum is 3. Blitzer, Intermediate Algebra, 5e – Slide #9 Section 5.4

Factoring Trinomials Factor:
EXAMPLE Factor: SOLUTION The GCF of the three terms of the polynomial is 4y. Therefore, we begin by factoring out 4y. Then we factor the remaining trinomial. Factor out the GCF Begin factoring Find two integers whose product is -18 and whose difference is 3. Difference 1 18 17 2 9 7 3 6 The integers are -3 and 6. Blitzer, Intermediate Algebra, 5e – Slide #10 Section 5.4

Factoring Trinomials A Strategy for Factoring T
Assume, for the moment, that there is no greatest common factor. 1) Find two First terms whose product is 2) Find two Last terms whose product is c: 3) By trial and error, perform steps 1 and 2 until the sum of the Outside product and Inside product is bx: Do I want to do this? I O Sum of O + I If no such combinations exist, the polynomial is prime. Blitzer, Intermediate Algebra, 5e – Slide #11 Section 5.4

Factoring Trinomials Factor:
Do I want to do this? Is there a limit to how many factors numbers can have? How about the number 24? EXAMPLE Factor: SOLUTION 1) Find two First terms whose product is There is more than one choice for our two First terms. Those choices are cataloged below. ? ? 2) Find two Last terms whose product is 15. There is more than one choice for our two Last terms. Those choices are cataloged below. ? ? Blitzer, Intermediate Algebra, 5e – Slide #12 Section 5.4

Factoring Trinomials Do I want to do this? CONTINUED 3) Try various combinations of these factors. The correct factorization of is the one in which the sum of the Outside and Inside products is equal to 19x. Here is a list of some of the possible factorizations. Possible Factorizations of Sum of Outside & Inside Products (Should Equal 19x) (6x + 1)(x + 15) 90x + x = 91x (x + 1)(6x + 15) 15x + 6x = 21x (3x + 3)(2x + 5) (2x + 3)(3x + 5) 10x + 9x = 19x (6x + 3)(x + 5) 30x + 3x = 33x (x + 3)(6x + 5) 5x + 18x = 23x Some means there are more This is the required middle term. Blitzer, Intermediate Algebra, 5e – Slide #13 Section 5.4

Factoring Trinomials Therefore, the factorization of is:
CONTINUED Therefore, the factorization of is: (2x + 3)(3x + 5) . Determine which possible factorizations were not represented in the chart on the preceding page. Do I want to do this? Blitzer, Intermediate Algebra, 5e – Slide #14 Section 5.4

Factoring Trinomials Factor by grouping: The trinomial is of the form
EXAMPLE using Grouping Factor by grouping: SOLUTION The trinomial is of the form a = 6 b = 19 c = 15 Multiply the leading coefficient, a, and the constant, c. Using a = 6 and c = 15. Blitzer, Intermediate Algebra, 5e – Slide #15 Section 5.4

Factoring Trinomials CONTINUED 2) Find the factors of ac whose difference is b. We want the factors of 90 whose sum is b, or 19. Sum 1 90 91 2 45 47 3 30 33 5 18 23 6 15 21 9 10 19 sum Matches 19 3) Rewrite the middle term. 19x = 9x+10x Blitzer, Intermediate Algebra, 5e – Slide #16 Section 5.4

Factoring Trinomials 4) Factor by grouping. Group terms
CONTINUED 4) Factor by grouping. Group terms Factor GCF from each group Factor out 2x +3, the common binomial factor Thus, Blitzer, Intermediate Algebra, 5e – Slide #17 Section 5.4

Factoring Trinomials Factor by grouping:
EXAMPLE using Grouping Factor by grouping: SOLUTION The GCF of the three terms of the polynomial is We begin by factoring out . a = 10 b =-17 c = 3 Multiply the leading coefficient, a, and the constant, c. Using a = 10 and c = 3. Blitzer, Intermediate Algebra, 5e – Slide #18 Section 5.4

Factoring Trinomials CONTINUED 2) Find the factors of ac whose difference is b. We want the factors of 30 whose sum is b, or 17. Sum 1 30 31 2 15 17 3 10 13 5 6 11 sum Matches 17 3) Rewrite the middle term. -17y = -2y-15y Blitzer, Intermediate Algebra, 5e – Slide #19 Section 5.4

Factoring Trinomials 4) Factor by grouping. Group terms
CONTINUED 4) Factor by grouping. Group terms Factor GCF from each group Factor out 2x +3, the common binomial factor Thus, Blitzer, Intermediate Algebra, 5e – Slide #20 Section 5.4

Factoring Trinomials Factor:
EXAMPLE Replace x cubed with t Factor: SOLUTION Notice that the exponent on is half that of the exponent on We will let t equal the variable to the power that is half of 6. Thus, let This is the given polynomial, with written as Let Rewrite the trinomial in terms of t. Factor the trinomial. Now substitute for t. Therefore, Blitzer, Intermediate Algebra, 5e – Slide #21 Section 5.4

Factoring Using Grouping T
Factoring Trinomials, page 349 Review step in book Factoring Using Grouping T 1) Multiply the leading coefficient, a, and the constant, c. 2) Find the factors of ac whose sum is b. 3) Rewrite the middle term, bx, as a sum or difference using the factors from step 2. 4) Factor by grouping. Blitzer, Intermediate Algebra, 5e – Slide #22 Section 5.4

Factoring Trinomials Factor by grouping: The trinomial is of the form
EXAMPLE Factor by grouping: SOLUTION The trinomial is of the form a = 1 b = 1 c = -12 1) Multiply the leading coefficient, a, and the constant, c. Using a = 1 and c = -12. Blitzer, Intermediate Algebra, 5e – Slide #23 Section 5.4

Factoring Trinomials CONTINUED 2) Find the factors of ac whose sum is b. We want the factors of -12 whose sum is b, or 1. The factors of -12 whose sum is 1 are 4 and -3. 3) Rewrite the middle term, a, as a sum or difference using the factors from step 2: 4 and -3. Blitzer, Intermediate Algebra, 5e – Slide #24 Section 5.4

Factoring Trinomials 4) Factor by grouping. Group terms
CONTINUED 4) Factor by grouping. Group terms Factor from each group Factor out a + 4, the common binomial factor Thus, Blitzer, Intermediate Algebra, 5e – Slide #25 Section 5.4

DONE

Factoring Trinomials In section 5.3, we factored certain polynomials having four terms using the method of grouping. Now, we will use trial and error and a problem solving process to factor trinomials. The polynomials we will begin with will have leading coefficients of one. We will begin by trying to factor these trinomials into a product of two binomials. Polynomials that cannot be factored over a given number set are said to be prime. Blitzer, Intermediate Algebra, 5e – Slide #27 Section 5.4

Factoring Trinomials Thus, CONTINUED Some Factors of -14 14, -1 7, -2
Sum of Factors 13 5 This is the desired sum. Thus, Blitzer, Intermediate Algebra, 5e – Slide #28 Section 5.4

Factoring Trinomials Factor:
EXAMPLE Factor: SOLUTION The GCF of the three terms of the polynomial is We begin by factoring out . 1) Find two First terms whose product is ? ? Blitzer, Intermediate Algebra, 5e – Slide #29 Section 5.4

Factoring Trinomials CONTINUED 2) Find two Last terms whose product is 3. The only possible factorization is (-1)(-3) since the sum of the Outside and Inside products must be -17y, having a negative coefficient. 3) Try various combinations of these factors. The correct factorization of is the one in which the sum of the Outside and Inside products is equal to -17y. A list of the possible factorizations can be found on the next page. Blitzer, Intermediate Algebra, 5e – Slide #30 Section 5.4

Factoring Trinomials CONTINUED Possible Factorizations of Sum of Outside & Inside Products (Should Equal -17y) (10y - 1)(y - 3) -30y - y = -31y (y - 1)(10y - 3) -3y - 10y = -13y (2y - 1)(5y - 3) -6y - 5y = -11y (5y - 1)(2y - 3) -15y - 2y = -17y This is the required middle term. The factorization of is (5y - 1)(2y - 3). Now we include the GCF in the complete factorization of the given polynomial. Thus, Blitzer, Intermediate Algebra, 5e – Slide #31 Section 5.4

Did not factor out the GCF
Factoring Trinomials Do I want to do this? EXAMPLE Factor: Did not factor out the GCF SOLUTION 1) Find two First terms whose product is ? ? ? The question marks inside the parentheses indicate that we are looking for the coefficients of y in each factor. Blitzer, Intermediate Algebra, 5e – Slide #32 Section 5.4

Factoring Trinomials Do I want to do this? CONTINUED 2) Find two Last terms whose product is -8. The possible factorizations are as follows. ? ? ? ? The question marks inside the parentheses indicate that we are looking for the coefficients of x in each factor. Blitzer, Intermediate Algebra, 5e – Slide #33 Section 5.4

Factoring Trinomials Do I want to do this? CONTINUED 3) Try various combinations of these factors. The correct factorization of is the one in which the sum of the Outside and Inside products is equal to 10xy. A list of the possible factorizations can be found below and on the next page. Possible Factorizations of Sum of Outside & Inside Products (Should Equal 10xy) (12x + 8y)(x - y) -12xy + 8xy = -4xy (x + 8y)(12x - y) -xy + 96xy = 95xy (12x + 4y)(x - 2y) -24xy + 4xy = -20xy (x + 4y)(12x - 2y) -2xy + 48xy = 46xy (6x + 8y)(2x - y) -6xy + 16xy = 10xy (2x + 8y)(6x - y) Blitzer, Intermediate Algebra, 5e – Slide #34 Section 5.4

Factoring Trinomials CONTINUED (6x + 4y)(2x - 2y) -12xy + 8xy = -4xy
Do I want to do this? CONTINUED Possible Factorizations of Sum of Outside & Inside Products (Should Equal 10xy) (6x + 4y)(2x - 2y) -12xy + 8xy = -4xy (2x + 4y)(6x - 2y) -4xy + 24xy = 20xy (4x + 8y)(3x - y) (3x + 8y)(4x - y) -3xy + 32xy = 29xy (4x + 4y)(3x - 2y) -8xy + 12xy = 4xy (3x + 4y)(4x - 2y) -6xy + 16xy = 10xy This is the required middle term. Only half of the possible factorizations were checked. For example, (12x – 8y)(x + y) was not checked. It was not necessary since the corresponding factorization (12x + 8y)(x - y) was checked and the absolute value of the coefficient, -4, was not 10. Blitzer, Intermediate Algebra, 5e – Slide #35 Section 5.4

Did not factor out the GCF, still there.
Factoring Trinomials Do I want to do this? CONTINUED Thus, Did not factor out the GCF, still there. NOTE: There was another factorization that resulted in the desired middle term, 10xy. That factorization would have worked just as well as the one selected above. That factorization was (6x + 8y)(2x - y) = (2)(3x +4y)(2x – y). The answer we got above is: (3x + 4y)(4x – 2y) = (3x+4y)(2x – y)(2). As you can see, these two answers are equivalent. This illustrates why the greatest common should always be factored out first. Doing this simplifies the factoring process for you. Take out here Blitzer, Intermediate Algebra, 5e – Slide #36 Section 5.4