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## Zeros of Polynomial Functions

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### Zeros of Polynomial Functions

Objectives:

Use the Fundamental Theorem of Algebra to determine the number of zeros of polynomial functions

Find rational zeros of polynomial functions

Find conjugate pairs of complex zeros

Find zeros of polynomials by factoring

Use Descartes’s Rule of Signs and the Upper and Lower Bound Rules to find zeros of polynomials

WHY???

Finding the zeros of a polynomial function can help you analyze the attendance at women’s college basketball games.

In the complex number system, every nth-degree polynomial has precisely “n” zeros.

- Fundamental Theorem of Algebra

If f(x) is a polynomial of degree n, where n > 0, then f has at least one zero in the complex number system

- Linear Factorization Theorem

If f(x) is a polynomial of degree n, where n > 0, then f has precisely n linear factors

where are complex numbers

Zeros of Polynomial Functions

- Give the degree of the polynomial, tell how many zeros there are, and find all the zeros

Rational Zero Test

- To use the Rational Zero Test, you should list all rational numbers whose numerators are factors of the constant term and whose denominators are factors of the leading coefficient
- Once you have all the possible zeros test them using substitution or synthetic division to see if they work and indeed are a zero of the function (Also, use a graph to help determine zeros to test)
- It only test for rational numbers

EXAMPLE: Using the Rational Zero Theorem

List all possible rational zeros of f(x)=15x3+ 14x2 - 3x – 2.

SolutionThe constant term is –2 and the leading coefficient is 15.

Divide 1

and 2

by 1.

Divide 1

and 2

by 3.

Divide 1

and 2

by 5.

Divide 1

and 2

by 15.

There are 16 possible rational zeros. The actual solution set to f(x)=15x3+ 14x2 - 3x – 2 = 0 is {-1, -1/3, 2/5}, which contains 3 of the 16 possible solutions.

Roots & Zeros of Polynomials II

Finding the Roots/Zeros of Polynomials (Degree 3 or higher):

- Graph the polynomial to find your first zero/root
- Use synthetic division to find a smaller polynomial
- If the polynomial is not a quadratic follow the 2 steps above using the smaller polynomial until you get a quadratic.
- Factor or use the quadratic formula to find your remaining zeros/roots

Find all the zeros of each polynomial function

First, graph the equation to find the first zero

ZERO

From looking at the graph you can see that there is a zero at -2

Second, use the zero you found from the graph and do synthetic division to find a smaller polynomial

-2

10 9 -19 6

Don’t forget your remainder should be zero

-20 22 -6

10 -11 3 0

The new, smaller polynomial is:

Third, factor or use the quadratic formula to find the remaining zeros.

This quadratic can be factored into: (5x – 3)(2x – 1)

Therefore, the zeros to the problem are:

Rational Zeros

- Find the rational zeros.

Writing a Polynomial given the zeros.

To write a polynomial you must write the zeros out in factored form. Then you multiply the factors together to get your polynomial.

Factored Form: (x – zero)(x – zero). . .

***If P is a polynomial function and a + bi is a root, then a – bi is also a root.

***If P is a polynomial function and is a root, then is also a root

The zeros of a third-degree polynomial are 2 (multiplicity 2) and -5. Write a polynomial.

First, write the zeros in factored form

(x – 2)(x – 2)(x – (-5)) = (x – 2)(x – 2)(x+5)

Second, multiply the factors out to find your polynomial

(x – 2)(x – 2)(x+5)

First FOIL or box two of the factors

Second, box your answer from above with your remaining factors to get your polynomial:

X

5

ANSWER

So if asked to find a polynomial that has zeros, 2 and 1 – 3i, you would know another root would be 1 + 3i. Let’s find such a polynomial by putting the roots in factor form and multiplying them together.

If x = the root then x - the root is the factor form.

Multiply the last two factors together. All i terms should disappear when simplified.

-1

Now multiply the x – 2 through

Here is a 3rd degree polynomial with roots 2, 1 - 3i and 1 + 3i

Conjugate Pairs

- Complex Zeros Occur in Conjugate Pairs = If a + bi is a zero of the function, the conjugate a – bi is also a zero of the function (the polynomial function must have real coefficients)
- EXAMPLES: Find a polynomial with the given zeros
- -1, -1, 3i, -3i
- 2, 4 + i, 4 – i

STEPS For Finding the Zeros given a Solution

- Find a polynomial with the given solutions (FOIL or BOX)
- Use long division to divide your polynomial you found in step 1 with your polynomial from the problem
- Factor or use the quadratic formula on the answer you found from long division.
- Write all of your answers out

If one root is 4 - i.

Find Roots/Zeros of a Polynomial

If the known root is imaginary, we can use the Complex Conjugates Thm.

Because of the Complex Conjugate Thm., we know that another root must be 4 + i.

If one root is 4 - i.

Example (con’t)

If one root is 4 - i, then one factor is [x - (4 - i)], and

Another root is 4 + i, & another factor is [x - (4 + i)].

Multiply these factors:

X

-4

-i

If the product of the two non-real factors is

Ex: Find all the roots of

then the third factor (that gives us the real root) is the quotient of P(x) divided by

If one root is 4 - i.

Example (con’t)

The third root is x = -3

So, all of the zeros are: 4 – i, 4 + i, and -3

Descartes’s Rule of Signs

- Let be a polynomial with real coefficients and
- The number of positive real zeros of f is either equal to the number of variations in sign of f(x) or less than that number by an even integer
- The number of negative real zeros of f is either equal to the number of variations in sign of f(-x) or less than that number by an even integer
- Variation in sign = two consecutive coefficients have opposite signs

1. To find possibilities for positive real zeros, count the number of sign changes in the equation for f(x).Because all the terms are positive, there are no variations in sign. Thus, there are no positive real zeros.

2. To find possibilities for negative real zeros, count the number of sign changes in the equation for f(-x). We obtain this equation by replacing x with -x in the given function.

f(x)= x3+ 2x2 + 5x + 4This is the given polynomial function.

Replace xwith -x.

f(-x)= (-x)3+2(-x)2+ 5(-x) + 4

=-x3+ 2x2 - 5x + 4

EXAMPLE:Using Descartes’ Rule of Signs

Determine the possible number of positive and negative real zeros of

f(x)= x3+ 2x2 + 5x + 4.

EXAMPLE:Using Descartes’ Rule of Signs

Determine the possible number of positive and negative real zeros of

f(x)= x3+ 2x2 + 5x + 4.

Solution

Now count the sign changes.

f(-x)=-x3+ 2x2 - 5x + 4

There are three variations in sign.

# of negative real zeros of fis either equal to 3, or is less than this number by an even integer.

This means that there are either 3 negative real zeros

or 3 - 2 = 1 negative real zero.

- EXAMPLES: describe the possible real zeros

Upper & Lower Bound Rules

- Let f(x) be a polynomial with real coefficients and a positive leading coefficient. Suppose f(x) is divided by x – c, using synthetic didvision
- If c > 0 and each number in the last row is either positive or zero, c is an upper bound for the real zeros of f
- If c < 0 and the numbers in the last row are alternately positive and negative (zero entries count as positive or negative), c is a lower bound for the real zeros of f
- EXAMPLE: find the real zeros

1 6 10 6 9

1

0

2

4

6

1

4

6

0

9

Signs are all positive, therefore 1 is an upper bound.

EXAMPLE

- You are designing candle-making kits. Each kit contains 25 cubic inches of candle wax and a model for making a pyramid-shaped candle. You want the height of the candle to be 2 inches less than the length of each side of the candle’s square base. What should the dimensions of your candle mold be?

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