Polynomial Functions and End Behavior

Understanding polynomial functions, their graphs, and how to determine end behavior

Polynomial Functions and End Behavior

What is a Polynomial Function?

A polynomial function is a function that can be written in the form:

f(x)=anxn+anโˆ’1xnโˆ’1+...+a1x+a0f(x) = a_n x^n + a_{n-1} x^{n-1} + ... + a_1 x + a_0

where:

  • nn is a non-negative integer (the degree of the polynomial)
  • a_n, a_{n-1, ..., a_0 are real numbers (the coefficients)
  • anโ‰ 0a_n \neq 0 (the leading coefficient)

Degree and Leading Coefficient

The degree is the highest power of xx in the polynomial.

The leading coefficient is the coefficient of the term with the highest power.

Example: f(x)=3x4โˆ’2x3+5xโˆ’7f(x) = 3x^4 - 2x^3 + 5x - 7

  • Degree: 4
  • Leading coefficient: 3

End Behavior

End behavior describes what happens to f(x)f(x) as xโ†’โˆžx \to \infty and xโ†’โˆ’โˆžx \to -\infty.

End Behavior Rules

The end behavior depends on:

  1. The degree (even or odd)
  2. The sign of the leading coefficient (positive or negative)

| Degree | Leading Coefficient | Left End (xโ†’โˆ’โˆž)(x \to -\infty) | Right End (xโ†’โˆž)(x \to \infty) | |--------|---------------------|----------------------------|---------------------------| | Even | Positive (+) | f(x)โ†’โˆžf(x) \to \infty | f(x)โ†’โˆžf(x) \to \infty | | Even | Negative (-) | f(x)โ†’โˆ’โˆžf(x) \to -\infty | f(x)โ†’โˆ’โˆžf(x) \to -\infty | | Odd | Positive (+) | f(x)โ†’โˆ’โˆžf(x) \to -\infty | f(x)โ†’โˆžf(x) \to \infty | | Odd | Negative (-) | f(x)โ†’โˆžf(x) \to \infty | f(x)โ†’โˆ’โˆžf(x) \to -\infty |

Key Concepts

  1. Even degree, positive leading coefficient: Both ends go up (U-shape)
  2. Even degree, negative leading coefficient: Both ends go down (upside-down U)
  3. Odd degree, positive leading coefficient: Left goes down, right goes up (/)
  4. Odd degree, negative leading coefficient: Left goes up, right goes down ()

Zeros and Multiplicity

The zeros (or roots) of a polynomial are the values of xx where f(x)=0f(x) = 0.

The multiplicity of a zero is how many times that factor appears.

  • Odd multiplicity: The graph crosses the x-axis
  • Even multiplicity: The graph touches the x-axis and turns around

Practice Strategy

To analyze a polynomial:

  1. Identify the degree and leading coefficient
  2. Determine the end behavior
  3. Find the zeros and their multiplicities
  4. Sketch the general shape of the graph

๐Ÿ“š Practice Problems

1Problem 1easy

โ“ Question:

Determine the end behavior of f(x)=โˆ’2x5+3x4โˆ’x+7f(x) = -2x^5 + 3x^4 - x + 7.

๐Ÿ’ก Show Solution

Solution:

Step 1: Identify the degree and leading coefficient.

  • Degree: 5 (odd)
  • Leading coefficient: -2 (negative)

Step 2: Apply the end behavior rules.

Since the degree is odd and the leading coefficient is negative:

  • As xโ†’โˆ’โˆžx \to -\infty, f(x)โ†’โˆžf(x) \to \infty (left end goes up)
  • As xโ†’โˆžx \to \infty, f(x)โ†’โˆ’โˆžf(x) \to -\infty (right end goes down)

Answer: limโกxโ†’โˆ’โˆžf(x)=โˆžย andย limโกxโ†’โˆžf(x)=โˆ’โˆž\lim_{x \to -\infty} f(x) = \infty \text{ and } \lim_{x \to \infty} f(x) = -\infty

2Problem 2easy

โ“ Question:

Consider the polynomial function f(x)=โˆ’2x5+3x4โˆ’x2+7f(x) = -2x^5 + 3x^4 - x^2 + 7.

a) What is the degree of this polynomial? b) What is the leading coefficient? c) Describe the end behavior of this function.

๐Ÿ’ก Show Solution

Solution:

Part (a): The degree is the highest power of xx in the polynomial.

The highest power is x5x^5, so the degree is 5.

Part (b): The leading coefficient is the coefficient of the term with the highest power.

The coefficient of x5x^5 is โˆ’2-2, so the leading coefficient is -2.

Part (c): For end behavior, we look at the degree and leading coefficient:

  • Degree: 5 (odd)
  • Leading coefficient: -2 (negative)

For a polynomial with odd degree and negative leading coefficient:

  • As xโ†’โˆžx \to \infty, f(x)โ†’โˆ’โˆžf(x) \to -\infty
  • As xโ†’โˆ’โˆžx \to -\infty, f(x)โ†’โˆžf(x) \to \infty

In words: The graph rises to the left and falls to the right.

3Problem 3medium

โ“ Question:

Given f(x)=x2(xโˆ’3)3(x+1)f(x) = x^2(x - 3)^3(x + 1), find all zeros and their multiplicities, then describe the graph behavior at each zero.

๐Ÿ’ก Show Solution

Solution:

Step 1: Find the zeros by setting each factor equal to zero.

  • x2=0โ‡’x=0x^2 = 0 \Rightarrow x = 0
  • (xโˆ’3)3=0โ‡’x=3(x - 3)^3 = 0 \Rightarrow x = 3
  • (x+1)=0โ‡’x=โˆ’1(x + 1) = 0 \Rightarrow x = -1

Step 2: Determine the multiplicity of each zero.

  • x=0x = 0: multiplicity 2 (even)
  • x=3x = 3: multiplicity 3 (odd)
  • x=โˆ’1x = -1: multiplicity 1 (odd)

Step 3: Describe behavior at each zero.

  • At x=0x = 0: The graph touches the x-axis and turns around (even multiplicity)
  • At x=3x = 3: The graph crosses the x-axis (odd multiplicity)
  • At x=โˆ’1x = -1: The graph crosses the x-axis (odd multiplicity)

Step 4: Determine overall end behavior.

  • Degree: 2+3+1=62 + 3 + 1 = 6 (even)
  • Leading coefficient: positive (from expanding, the leading term is x6x^6)
  • End behavior: Both ends go to โˆž\infty

Answer: Zeros at x=โˆ’1x = -1 (crosses), x=0x = 0 (touches), and x=3x = 3 (crosses)

4Problem 4medium

โ“ Question:

A polynomial function g(x)g(x) has the following properties:

  • Degree 4
  • Leading coefficient is positive
  • Has zeros at x=โˆ’2,1,3x = -2, 1, 3 (each with multiplicity 1)
  • Passes through the point (0,โˆ’12)(0, -12)

a) Write a general form for g(x)g(x) using the known zeros. b) Find the specific equation for g(x)g(x). c) Describe the end behavior.

๐Ÿ’ก Show Solution

Solution:

Part (a): With zeros at x=โˆ’2,1,3x = -2, 1, 3, the function has factors (x+2)(x + 2), (xโˆ’1)(x - 1), and (xโˆ’3)(x - 3).

Since the degree is 4 and we only have 3 zeros (each multiplicity 1), we need one more factor. Let's call it (xโˆ’r)(x - r) where rr is unknown.

General form: g(x)=a(x+2)(xโˆ’1)(xโˆ’3)(xโˆ’r)g(x) = a(x + 2)(x - 1)(x - 3)(x - r)

where aa is a positive constant.

Part (b): We use the point (0,โˆ’12)(0, -12) to find aa and rr.

However, with two unknowns and one equation, we need more information. Let's assume the fourth zero is actually a repeated zero. If x=โˆ’2x = -2 has multiplicity 2:

g(x)=a(x+2)2(xโˆ’1)(xโˆ’3)g(x) = a(x + 2)^2(x - 1)(x - 3)

Substituting (0,โˆ’12)(0, -12): โˆ’12=a(2)2(โˆ’1)(โˆ’3)-12 = a(2)^2(-1)(-3) โˆ’12=a(4)(3)-12 = a(4)(3) โˆ’12=12a-12 = 12a a=โˆ’1a = -1

But we need aa positive! So let's try x=1x = 1 with multiplicity 2:

g(x)=a(x+2)(xโˆ’1)2(xโˆ’3)g(x) = a(x + 2)(x - 1)^2(x - 3)

โˆ’12=a(2)(1)2(โˆ’3)-12 = a(2)(1)^2(-3) โˆ’12=a(2)(โˆ’3)=โˆ’6a-12 = a(2)(-3) = -6a a=2a = 2

Therefore: g(x)=2(x+2)(xโˆ’1)2(xโˆ’3)g(x) = 2(x + 2)(x - 1)^2(x - 3)

Part (c): Degree 4 (even), positive leading coefficient:

  • As xโ†’โˆžx \to \infty, g(x)โ†’โˆžg(x) \to \infty
  • As xโ†’โˆ’โˆžx \to -\infty, g(x)โ†’โˆžg(x) \to \infty

The graph rises on both ends.

5Problem 5medium

โ“ Question:

Write a polynomial function with zeros at x=โˆ’2x = -2 (multiplicity 2), x=1x = 1 (multiplicity 1), and x=4x = 4 (multiplicity 1), and end behavior where f(x)โ†’โˆ’โˆžf(x) \to -\infty as xโ†’ยฑโˆžx \to \pm\infty.

๐Ÿ’ก Show Solution

Solution:

Step 1: Write the polynomial in factored form using the zeros.

With zeros at x=โˆ’2x = -2 (mult. 2), x=1x = 1 (mult. 1), and x=4x = 4 (mult. 1): f(x)=a(x+2)2(xโˆ’1)(xโˆ’4)f(x) = a(x + 2)^2(x - 1)(x - 4)

Step 2: Determine the sign of the leading coefficient.

Total degree: 2+1+1=42 + 1 + 1 = 4 (even)

For both ends to go to โˆ’โˆž-\infty with an even degree, we need a negative leading coefficient.

So a<0a < 0. We can choose a=โˆ’1a = -1.

f(x)=โˆ’(x+2)2(xโˆ’1)(xโˆ’4)f(x) = -(x + 2)^2(x - 1)(x - 4)

Step 3: Verify (optional - expand to check).

If we expand: f(x)=โˆ’(x+2)2(x2โˆ’5x+4)f(x) = -(x + 2)^2(x^2 - 5x + 4)

The leading term will be โˆ’x4-x^4, confirming our negative leading coefficient.

Answer: f(x)=โˆ’(x+2)2(xโˆ’1)(xโˆ’4)f(x) = -(x + 2)^2(x - 1)(x - 4) or any negative constant multiple