# 5.5 Real Zeros of Polynomial Functions

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5.5 Real Zeros of Polynomial Functions
MAT SP 2008 5.5 Real Zeros of Polynomial Functions In this section, we will study the following topics: The Remainder and Factor Theorems The Rational Zeros Theorem Finding the Real Zeros of a Polynomial Function Solving Polynomial Equations The Intermediate Value Theorem

Review Topics Polynomial Long Division (pp 44-47)
MAT SP 2008 Review Topics For the next two sections, you will need to know the following: Polynomial Long Division (pp 44-47) Synthetic Division (pp 57-60) Quadratic Formula (pp )

Review: Long Division of Polynomials
MAT SP 2008 Review: Long Division of Polynomials Example

Long Division of Polynomials
MAT SP 2008 Long Division of Polynomials Remember, for every division problem the following statement is true: Dividend = (divisor) x (quotient) + remainder For polynomial division, we express the division algorithm as:

Long Division of Polynomials
MAT SP 2008 Long Division of Polynomials Using the previous example, we have

Review: Synthetic Division
MAT SP 2008 Review: Synthetic Division Synthetic division is a really cool shortcut for dividing polynomials by divisors in the form x – c. As with long division, before you start synthetic division, first do the following: Arrange the terms in descending order. Use zero placeholders, where necessary. NOTE: If you are dividing by the binomial x - c, then you would use c as the divisor. Likewise, if you are dividing by x + c, then you would use (- c) as the divisor, since x + c = x - (- c).

Synthetic Division Example #1 Divide The divisor is x – 3, so c =
MAT SP 2008 Synthetic Division Example #1 Divide The divisor is x – 3, so c = Arranging the terms of the dividend in descending order we have: So, you would set this synthetic division problem up as: c Coefficients of dividend

Synthetic Division Example #1 (cont) Now we are ready to divide.
MAT SP 2008 Synthetic Division Example #1 (cont) Now we are ready to divide.

Synthetic Division (continued)
MAT SP 2008 Synthetic Division (continued) Example #2 Use synthetic division to divide:

Quadratic Formula To solve equations in the form:

The Remainder Theorem The Remainder Theorem
MAT SP 2008 The Remainder Theorem The Remainder Theorem If a polynomial f(x) is divided by x – c, the remainder r is the same as the value of f(c). This means that you can evaluate a polynomial function at a given value by substituting the value into the function, OR -using synthetic division to find the remainder.

Exciting new synthetic method:
MAT SP 2008 The Remainder Theorem Example Find f(5) given f(x) = 4x2 –10x – 21 Old, boring method: Exciting new synthetic method: f(5) = 4(5)2 –10(5) – 21 f(5) =

The Factor Theorem The Factor Theorem
MAT SP 2008 The Factor Theorem The Factor Theorem If a polynomial f(c) has a factor (x – c) iff f(c) = 0. This theorem tells us that, in order for x – c to be a factor of the polynomial, the remainder when the polynomial is divided by x – c must be zero. That makes sense... For example, 8 is a factor of 32 since 8 divides into 32 evenly (the remainder is zero.)

The Importance of the Remainder Theorem
MAT SP 2008 The Importance of the Remainder Theorem If f(x) is divided by x – c and the remainder is equal to 0, then (x – c) is a factor of f(x) If c is a real number, (c, 0) is an x-intercept of the graph of f. So, from this graph of f(x), we can determine:

MAT SP 2008

The Rational Zero Theorem
MAT SP 2008 The Rational Zero Theorem The Rational Zero Theorem If the polynomial f(x)=anxn + an-1xn-1 + … + a2x2 + a1x + a0 has integer coefficients, then every rational zero must have the form , where p IS A FACTOR OF THE CONSTANT TERM a0 , and q IS A FACTOR OF THE LEADING COEFFICIENT an. This theorem will enable us to list all of the potential rational zeros of a polynomial, using the form

Factors of the constant
MAT SP 2008 Factors of the constant Factors of the leading coefficient

The Rational Zero Test Example Find all potential rational zeros of
MAT SP 2008 The Rational Zero Test Example Find all potential rational zeros of Solution

The Rational Zero Test (continued)
MAT SP 2008 The Rational Zero Test (continued) Once you have the list of all potential rational zeros, you need to use trial and error to test them using synthetic division (or by substituting them into the function) to determine which ones are actual zeros. Remember, the remainder (or the functional value) must be equal to zero. A Sneaky Technology Shortcut: You can use the graph or the table of values to find one or more of the rational zeros, if there are any. Use the fact that a real zero is an x-intercept of the graph. Use this zero to perform synthetic division. A zero remainder will confirm that it is an actual zero.

The Rational Zero Test (continued)
MAT SP 2008 The Rational Zero Test (continued) Example Use the Rational Zero Test to find ALL rational zeros of

Using the Quotient to find the remaining zeros
MAT SP 2008 Using the Quotient to find the remaining zeros Each time you divide a polynomial using synthetic division, the QUOTIENT HAS A DEGREE THAT IS ONE LESS THAN THE ORIGINAL POLYNOMIAL. We will use the resulting lower-degree (“depressed”) polynomial find the remaining zeros. Your goal is obtain a lower-degree polynomial that is quadratic. Then you can find the remaining zeros by factoring or using the quadratic formula.

Scenario 1: The original polynomial is CUBIC
MAT SP 2008 Scenario 1: The original polynomial is CUBIC FIND ONE RATIONAL ZERO, using the Rational Zero Theorem to find potential rational zeros and then using the graph to help you locate one rational zero. DIVIDE THE CUBIC POLYNOMIAL BY THE RATIONAL ZERO using synthetic division. SOLVE THE DEPRESSED QUADRATIC EQUATION to find the remaining zeros (by factoring, completing the square, or quadratic formula).

MAT SP 2008 Example #1 Find all real zeros of algebraically.

MAT SP 2008 Example #2 Find all real zeros of algebraically.

Scenario 2: The original polynomial is QUARTIC
MAT SP 2008 Scenario 2: The original polynomial is QUARTIC FIND TWO RATIONAL ZEROS, using the rational zero test to find potential rational zeros and then using the graph to help you locate TWO rational zeros. (Be on the alert for double zeros!) DIVIDE THE QUARTIC POLYNOMIAL BY ONE OF THE RATIONAL ZEROS using synthetic division. DIVIDE THE DEPRESSED CUBIC POLYNOMIAL BY THE OTHER RATIONAL ZERO using synthetic division. SOLVE THE DEPRESSED QUADRATIC EQUATION to find the remaining zeros (by factoring, completing the square, or quadratic formula).

MAT SP 2008 Example #1 Find all real zeros of algebraically.

MAT SP 2008

MAT SP 2008 Example #2 Show that -5 and 3 are zeros of f(x) and use this information to write the complete factorization of f.

Approximating Zeros of Polynomial Functions
MAT SP 2008 Approximating Zeros of Polynomial Functions Often we are not able to find the zeros of a polynomial function algebraically, but we can still approximate the value of the zero. We know that the graphs of polynomials are continuous; therefore, if the sign of the function values (y-values) changes from negative to positive or vice-versa, we know that the graph must have passed through the x-axis and hence, has a zero in that interval. This result stems from the INTERMEDIATE VALUE THEOREM.

MAT SP 2008

Approximating Zeros of Polynomial Functions
MAT SP 2008 Approximating Zeros of Polynomial Functions We can use the table of values to find the intervals (of length 1) in which a polynomial function is guaranteed to have a zero. Example: The following is a table of polynomial function values. The functional (y) values change: from + to – in the interval -4 < x < -3, from – to + in the interval –1 < x < 0, from + to – in the interval 0 < x < 1. So this polynomial function is guaranteed to have a zero in each of the following intervals: (-4, -3), (-1, 0), and (0, 1).

Approximating Zeros of Polynomial Functions
MAT SP 2008 Approximating Zeros of Polynomial Functions Example: Use the table of values on your calculator to find the intervals (of length 1) in which the function is guaranteed to have a zero.

MAT SP 2008 End of Sect. 5.5