§ 5.4 Factoring Trinomials

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, Algebra for College Students, 6e – Slide #2 Section 5.4

A Strategy for Factoring T Factoring Trinomials 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, Algebra for College Students, 6e – Slide #3 Section 5.4

Factoring Trinomials Factor: EXAMPLE Factor: 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. Some Factors of 24 24, 1 -24, -1 12, 2 -12, -2 8, 3 -8, -3 6, 4 -6, -4 Blitzer, Algebra for College Students, 6e – 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. Blitzer, Algebra for College Students, 6e – 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, Algebra for College Students, 6e – Slide #6 Section 5.4

Factoring Trinomials Factor: EXAMPLE Factor: SOLUTION 1) Enter a as the first term of each factor. To find the second term of each factor, we must find two integers whose product is -14 and whose sum is 5. 2) & 3) List pairs of factors of the constant, -14, and try various combinations of these factors. Because the desired sum, 5, is positive, the positive factor of -14 must be farther from 0 than the negative factor is. Thus, we will only list pairs of factors of -14 in which the positive factor has the larger absolute value. Blitzer, Algebra for College Students, 6e – Slide #7 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, Algebra for College Students, 6e – Slide #8 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 sum is 3. The integers are -3 and 6. Thus, Blitzer, Algebra for College Students, 6e – Slide #9 Section 5.4

A Strategy for Factoring T 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: I O Sum of O + I If no such combinations exist, the polynomial is prime. Blitzer, Algebra for College Students, 6e – Slide #10 Section 5.4

Factoring Trinomials Factor: 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, Algebra for College Students, 6e – Slide #11 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 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 This is the required middle term. Blitzer, Algebra for College Students, 6e – Slide #12 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 slide. Blitzer, Algebra for College Students, 6e – Slide #13 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, Algebra for College Students, 6e – Slide #14 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 slide. Blitzer, Algebra for College Students, 6e – Slide #15 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, Algebra for College Students, 6e – Slide #16 Section 5.4

Factoring Trinomials Factor: 1) Find two First terms whose product is EXAMPLE Factor: 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, Algebra for College Students, 6e – Slide #17 Section 5.4

Factoring Trinomials 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, Algebra for College Students, 6e – Slide #18 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 10xy. A list of the possible factorizations can be found below and on the next slide. 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, Algebra for College Students, 6e – Slide #19 Section 5.4

Factoring Trinomials CONTINUED (6x + 4y)(2x - 2y) -12xy + 8xy = -4xy 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, Algebra for College Students, 6e – Slide #20 Section 5.4

Factoring Trinomials Thus, CONTINUED Thus, 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. Blitzer, Algebra for College Students, 6e – Slide #21 Section 5.4

Factoring Trinomials Factor: EXAMPLE 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, Algebra for College Students, 6e – Slide #22 Section 5.4

Factoring Using Grouping T Factoring Trinomials 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, Algebra for College Students, 6e – Slide #23 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, Algebra for College Students, 6e – Slide #24 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, Algebra for College Students, 6e – Slide #25 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, Algebra for College Students, 6e – Slide #26 Section 5.4