1. Warm Up Factor completely. 1. 2y3 + 4y2 – 30y 2y(y – 3)(y + 5)
2. 3x4 – 6x2 – 24
3(x – 2)(x + 2)(x2 + 2)
Solve each equation.
3. x2 – 9 = 0
x = – 3, 3
4. x3 + 3x2 – 4x = 0
x = –4, 0, 1

2. Objectives Identify the multiplicity of roots.
Use the Rational Root Theorem and the irrational Root Theorem to solve polynomial equations.

3. Vocabulary
multiplicity

4. “Roots/zeros/solutions” of polynomials are points
where the function equals 0. Aka, the x-intercepts
Finding the roots of a polynomials
means identifying its x-intercepts

9. In Lesson 6-4, you used several methods for factoring polynomials
In Lesson 6-4, you used several methods for factoring polynomials. As with some quadratic equations, factoring a polynomial equation is one way to find its real roots.
Recall the Zero Product Property from Lesson 5-3. You can find the roots, or solutions, of the polynomial equation P(x) = 0 by setting each factor equal to 0 and solving for x.

10. Finding solutions of already factored polynomials
Ex:
Ex:

13. Note: You may have to use multiple
methods of factoring in a single problem

14. Example 1A: Using Factoring to Solve Polynomial Equations
Solve the polynomial equation by factoring.
4x6 + 4x5 – 24x4 = 0
4x4(x2 + x – 6) = 0
Factor out the GCF, 4x4.
4x4(x + 3)(x – 2) = 0
Factor the quadratic.
4x4 = 0 or (x + 3) = 0 or (x – 2) = 0
Set each factor equal to 0.
x = 0, x = –3, x = 2
Solve for x.
The roots are 0, –3, and 2.

15. Example 1A Continued
Check Use a graph. The roots appear to be located at x = 0, x = –3, and x = 2. 

16. Example 1B: Using Factoring to Solve Polynomial Equations
Solve the polynomial equation by factoring.
x = 26x2
x4 – 26 x = 0
Set the equation equal to 0.
Factor the trinomial in quadratic form.
(x2 – 25)(x2 – 1) = 0
(x – 5)(x + 5)(x – 1)(x + 1)
Factor the difference of two squares.
x – 5 = 0, x + 5 = 0, x – 1 = 0, or x + 1 =0
x = 5, x = –5, x = 1 or x = –1
Solve for x.
The roots are 5, –5, 1, and –1.

17. Check It Out! Example 1a
Solve the polynomial equation by factoring.
2x6 – 10x5 – 12x4 = 0
2x4(x2 – 5x – 6) = 0
Factor out the GCF, 2x4.
2x4(x – 6)(x + 1) = 0
Factor the quadratic.
2x4 = 0 or (x – 6) = 0 or (x + 1) = 0
Set each factor equal to 0.
x = 0, x = 6, x = –1
Solve for x.
The roots are 0, 6, and –1.

18. Check It Out! Example 1b
Solve the polynomial equation by factoring.
x3 – 2x2 – 25x = –50
x3 – 2x2 – 25x + 50 = 0
Set the equation equal to 0.
(x + 5)(x – 2)(x – 5) = 0
Factor.
x + 5 = 0, x – 2 = 0, or x – 5 = 0
x = –5, x = 2, or x = 5
Solve for x.
The roots are –5, 2, and 5.

21. Sometimes a polynomial equation has a factor that appears more than once. This creates a multiple root. In 3x5 + 18x4 + 27x3 = 0 has two multiple roots, 0 and –3. For example, the root 0 is a factor three times because 3x3 = 0.
The multiplicity of root r is the number of times that x – r is a factor of P(x). When a real root has even multiplicity, the graph of y = P(x) touches the x-axis but does not cross it. When a real root has odd multiplicity greater than 1, the graph “bends” as it crosses the x-axis.

22. You cannot always determine the multiplicity of a root from a graph
You cannot always determine the multiplicity of a root from a graph. It is easiest to determine multiplicity when the polynomial is in factored form.

25. Warm Up: Identify the roots
1. 2. x y -1 4 2 5 8 -3

26. Identify the Roots
Methods: Graph, Plysmlt2, Factor

27. Linear Factors
Use synthetic division to confirm the linear factors of example 1.

28. Identify the Roots
Methods: Graph, Plysmlt2, Factor

29. Linear Factors
Use synthetic division to confirm the linear factors of example 2.

30. Identify the Roots
Methods: Graph, Plysmlt2, Factor

31. Linear Factors
Use synthetic division to confirm the linear factors of example 3. Then use quadratic formula to confirm the other roots.

32. Identify the Roots
Methods: Graph, Plysmlt2, Factor

33. Linear Factors
Use synthetic division to confirm the linear factors of example 4. Then use quadratic formula to confirm the other roots.

34. Example 2A: Identifying Multiplicity
Identify the roots of each equation. State the multiplicity of each root.
x3 + 6x2 + 12x + 8 = 0
x3 + 6x2 + 12x + 8 = (x + 2)(x + 2)(x + 2)
x + 2 is a factor three times. The root –2 has a multiplicity of 3.
Check Use a graph. A calculator graph shows a bend near (–2, 0). 

35. Example 2B: Identifying Multiplicity
Identify the roots of each equation. State the multiplicity of each root.
x4 + 8x3 + 18x2 – 27 = 0
x4 + 8x3 + 18x2 – 27 = (x – 1)(x + 3)(x + 3)(x + 3)
x – 1 is a factor once, and x + 3 is a factor three times. The root 1 has a multiplicity of 1. The root –3 has a multiplicity of 3.
Check Use a graph. A calculator graph shows a bend near (–3, 0) and crosses at (1, 0). 

36. x – 2 is a factor four times. The root 2 has a multiplicity of 4.
Check It Out! Example 2a
Identify the roots of each equation. State the multiplicity of each root.
x4 – 8x3 + 24x2 – 32x + 16 = 0
x4 – 8x3 + 24x2 – 32x + 16 = (x – 2)(x – 2)(x – 2)(x – 2)
x – 2 is a factor four times. The root 2 has a multiplicity of 4.
Check Use a graph. A calculator graph shows a bend near (2, 0). 

37. Check It Out! Example 2b
Identify the roots of each equation. State the multiplicity of each root.
2x6 – 22x5 + 48x4 + 72x3 = 0
2x6 – 22x5 + 48x4 + 72x3 = 2x3(x + 1)(x – 6)(x – 6)
x is a factor three times, x + 1 is a factor once, and x – 6 is a factor two times.
The root 0 has a multiplicity of 3. The root –1 has a multiplicity of 1. The root 6 has a multiplicity of 2.

38. Not all polynomials are factorable, but the Rational Root Theorem can help you find all possible rational roots of a polynomial equation.

39. Example 3: Marketing Application
The design of a box specifies that its length is 4 inches greater than its width. The height is 1 inch less than the width. The volume of the box is 12 cubic inches. What is the width of the box?
Step 1 Write an equation to model the volume of the box.
Let x represent the width in inches. Then the length is x + 4, and the height is x – 1.
x(x + 4)(x – 1) = 12
V = lwh.
x3 + 3x2 – 4x = 12
Multiply the left side.
x3 + 3x2 – 4x – 12 = 0
Set the equation equal to 0.

40. Example 3 Continued
Step 2 Use the Rational Root Theorem to identify all possible rational roots.
Factors of –12: ±1, ±2, ±3, ±4, ±6, ±12
Step 3 Test the possible roots to find one that is actually a root. The width must be positive, so try only positive rational roots.
Use a synthetic substitution table to organize your work. The first row represents the coefficients of the polynomial. The first column represents the divisors and the last column represents the remainders. Test divisors to identify at least one root.

41. Example 3 Continued
Step 4 Factor the polynomial. The synthetic substitution of 2 results in a remainder of 0, so 2 is a root and the polynomial in factored form is (x – 2)(x2 + 5x + 6).
(x – 2)(x2 + 5x + 6) = 0
Set the equation equal to 0.
(x – 2)(x + 2)(x + 3) = 0
Factor x2 + 5x + 6.
Set each factor equal to 0, and solve.
x = 2, x = –2, or x = –3
The width must be positive, so the width should be 2 inches.

42. Lesson Quiz
Solve by factoring.
1. x3 + 9 = x2 + 9x
–3, 3, 1
Identify the roots of each equation. State the multiplicity of each root.
0 and 2 each with multiplicity 2
2. 5x4 – 20x3 + 20x2 = 0
3. x3 – 12x2 + 48x – 64 = 0
4 with multiplicity 3
4. A box is 2 inches longer than its height. The width is 2 inches less than the height. The volume of the box is 15 cubic inches. How tall is the box?
3 in.
5. Identify all the real roots of x3 + 5x2 – 3x – 3 = 0.
1, ,