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Section0.6Powers, Polynomials, and Rational Functions

Objectives
  • What are power, polynomial, and rational functions?

  • How can be build polynomial functions from data?

  • What microscopic/macroscopic behavior can we expect from polynomial and rational functions?

Polynomial functions play an important role in mathematics. They are generally simple to compute (requiring only computations that can be done by hand) and can be used to model many real-world phenomena. In fact, scientists and mathematicians frequently simplify complex mathematical models by substituting a polynomial model that is "close enough" for their purposes.

In this section, we will study the graphs of select polynomial and rational functions to identify their important features. The goal of this section is to build a mathematical intuition about how a small class of 'convenient' functions behave so that later we can see how calculus can be used to determine the behavior of arbitrary functions.

Preview Activity0.6.1

Figure 1 shows the graphs of two different functions. Suppose that you were to graph a line anywhere along each of the two graphs.

  1. Is it possible to draw a line that does not intersect the graph of \(f(x)\text{?}\) \(g(x)\text{?}\)

  2. What is the fewest number of intersections that your line could have with the graph of \(f(x)\text{?}\) with \(g\text{?}\)

  3. What is the largest number of intersections that your line could have with the graph of \(f(x)\text{?}\) with \(g(x)\text{?}\)

  4. How many times does the graph of \(f(x)\) change directions? How many times does the graph of \(g(x)\) change directions?

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Figure0.6.1\(f(x)\) and \(g(x)\) for the preview activity.

Subsection0.6.1Power Functions

Power functions are fundamental building blocks for many very useful functions. In their simplest form, power functions describe situations when the dependent variable is directly proportional to a power of the independent variable.

Power Functions

A power function has the form

\begin{equation*} f(x) = kx^{n}, \qquad \mbox{where } k \mbox{ and } n \mbox{ are constant.} \end{equation*}

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Figure0.6.3Positive odd powers of \(x\) (left) and positive even powers of \(x\) (right)
Features of Power Functions
\begin{equation*} f(x) = x^n \end{equation*}
  • Odd values of \(n\):
    • the graphs of \(x^{n}\) are always increasing
    • left end behavior: \(f(x) \to -\infty\) as \(x\to-\infty\)
    • right end behavior: \(f(x) \to +\infty\) as \(x\to+\infty\text{.}\)
    • graphs of odd power functions go in opposite directions on the left and right
  • Even values of \(n\):
    • the graphs of \(x^{n}\) decrease until \(x=0\) and then increase afterwards,
    • left end behavior: \(f(x) \to +\infty\) as \(x\to-\infty\)
    • right end behavior: \(f(x) \to +\infty\) as \(x\to+\infty\text{.}\)
    • graphs of even power functions go in the same direction on the left and right
Activity0.6.2

Power functions and exponential functions appear somewhat similar in their formulas, but behave differently in many ways.

  1. Compare the functions \(f(x)=x^2\) and \(g(x)=2^x\) by graphing both functions in several viewing windows. Find the points of intersection of the graphs. Which function grows more rapidly when \(x\) is large?

  2. Compare the functions \(f(x)=x^{10}\) and \(g(x)=2^x\) by graphing both functions in several viewing windows. Find the points of intersection of the graphs. Which function grows more rapidly when \(x\) is large?

  3. Make a conjecture: As \(x\to\infty\), which dominates, \(x^n\) or \(a^x\) for \(a>1\)?

Hint
Solution

Subsection0.6.2Polynomial Functions

Power functions have very predictable behavior but when we add or subtract several power functions we can model much more complicated behavior. A function made out of the sum of several power functions is known as a polynomial.
Polynomial Functions

A polynomial function is a function of the form

\begin{equation*} p(x)=a_{n}x^{n} + a_{n-1}x^{n-1}+\cdots + a_{1}x + a_{0} \end{equation*}

where \(n\) is a nonnegative integer and \(a_{n}\ne 0\text{.}\) Here \(n\) is the degree of the polynomial, and \(a_{n}\text{,}\) \(a_{n-1}\text{,}\) …, \(a_{1}\text{,}\) \(a_{0}\) are the coefficients.

When we study the graphs of functions, there are several common features we're interested in.

  • How does the graph behave as \(x\to\infty\) and as \(x\to-\infty\text{?}\) (Does it grow without bound? Does it level off? Does it oscillate?)

  • Where does the graph cross the \(x\)-axis? How many times?

  • Where does the graph change directions? How many times?

For many functions, these questions can be difficult to answer and require specialized mathematics (like Calculus for example). For polynomials, though, there are some relatively simple results. First, the end behavior of a polynomial is determined by its degree and the sign of the lead coefficient. Polynomials with even degree behave like power functions with even degree, and polynomials with odd degree behave like power functions like odd degree. Figures 5 and Figure 6 demonstrate this for two different fourth degree polynomials.

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Figure0.6.5Local behavior of two fourth-degree polynomials. At this level, we can clearly see the differences between these two functions.

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Figure0.6.6Long-term behavior of two fourth-degree polynomials. At this scale, the two functions are nearly indistinguishable.
Zeros and Turning Points of Polynomials

A polynomial of degree \(n\) has at most \(n\) real zeros and at most \(n-1\) turning points.

Zeros and Factors of Polynomials

Let \(p(x)\) be a polynomial. If \(x = a\) is a zero of \(p\) (i.e. \(p(a)=0\)), then \((x-a)\) is a factor of \(p\text{.}\)

The two statements above are each fairly simple to state but very powerful. The first gives us a quick way to determine the degree of a polynomial from its graph and is frequently used to determine how many solutions to expect from certain types of equations. The second provides us with a way to construct polynomials that pass through specific points.

Example 0.6.1

Write a polynomial function that has zeros at \(x=2, -3,\) and \(7\) and goes through the point \((1,3)\).

Solution
Example 0.6.2

The polynomial \(p(x)\) has degree 3 and passes through the points \((0,5)\), \((1,-2)\), \((3,0)\), and \((-1,4)\). Find \(p(x)\).

Solution
Activity0.6.3

For each of the following graphs, find a possible formula for the polynomial of lowest degree that fits the graph.

Hint
Solution

Subsection0.6.3Rational Functions

Rational Functions

A rational function is a ratio of two polynomial functions

\begin{equation*} f(x) = \frac{P(x)}{Q(x)} \end{equation*}

where \(P\) and \(Q\) are polynomials. The domain is the set of all real numbers for which \(Q(x)\ne 0\)

Horizontal Asymptotes

A function \(f\) has a horizontal asymptote \(y = a\) if the distance between the \(f\) and the line \(y=a\) becomes arbitrarily small when \(x\) becomes sufficiently large. Alternatively, a horizontal asymptote is a line that a function approaches as \(x \to \infty\) or as \(x \to -\infty\text{.}\) It should be noted that horizontal asymptotes refer to the end behavior of a rational function and rational functions can certainly cross horizontal asymptotes.

Vertical Asymptotes

A function \(f\) has a vertical asymptote at a point \(x = b\) if the function becomes arbitrarily large as \(x \to b\text{.}\)

The graphs of rational functions may have vertical asymptotes only where the denominator is zero. However, there are many examples of rational functions that do not have a vertical asymptote even at a point where the denominator is zero. (For instance, try graphing the function \(f(x)=\displaystyle{\frac{x^{2}-1}{x-1}}\)).

Activity0.6.4
  1. Suppose \(f(x) = x^{2}+ 3x + 2\) and \(g(x) = x - 3\).

    1. What is the behavior of the function \(h(x) = \displaystyle{\frac {f(x)}{g(x)}}\) near \(x = -1\)? (i.e. what happens to \(h(x)\) as \(x\to -1\)?) near \(x = -2\)? near \(x = 3\)?

    2. What is the behavior of the function \(k(x) = \displaystyle{\frac {g(x)}{f(x)}}\) near \(x = -1\)? near \(x = -2\)? near \(x = 3\)?

  2. Suppose \(f(x) = x^{2} - 9\) and \(g(x) = x - 3\).

    1. What is the behavior of the function \(h(x) = \displaystyle{\frac {f(x)}{g(x)}}\) near \(x = -3\)? (i.e. what happens to \(h(x)\) as \(x\to -3\)?) near \(x = 3\)?

    2. What is the behavior of the function \(k(x) = \displaystyle{\frac {g(x)}{f(x)}}\) near \(x = -3\)? near \(x = 3\)?

Hint
Solution
Activity0.6.5
  1. Suppose \(f(x) = x^{3} + 2x^{2}-x + 7\) and \(g(x) = x^{2} + 4x + 2\).

    1. Which function dominates as \(x \to \infty\)?

    2. What is the behavior of the function \(h(x) = \displaystyle{\frac {f(x)}{g(x)}}\) as \(x \to \infty\)?

    3. What is the behavior of the function \(k(x) = \displaystyle{\frac {g(x)}{f(x)}}\) as \(x \to \infty\)?

  2. Suppose \(f(x) = 2x^{4} - 5x^{3} + 8x^{2} - 3x - 1\) and \(g(x) = 3x^{4} - 2x^{2} + 1\)

    1. Which function dominates as \(x \to \infty\)?

    2. What is the behavior of the function \(h(x) = \displaystyle{\frac {f(x)}{g(x)}}\) as \(x \to \infty\)?

    3. What is the behavior of the function \(k(x) = \displaystyle{\frac {g(x)}{f(x)}}\) as \(x \to \infty\)?

  3. Suppose \(f(x) = e^{x}\) and \(g(x) = x^{10}\).

    1. Which function dominates as \(x \to \infty\) as \(x \to \infty\)?

    2. What is the behavior of the function \(h(x) = \displaystyle{\frac {f(x)}{g(x)}}\) as \(x \to \infty\)?

    3. What is the behavior of the function \(k(x) = \displaystyle{\frac {g(x)}{f(x)}}\) as \(x \to \infty\)?

Hint
Solution
Activity0.6.6

For each of the following functions, determine (1) whether the function has a horizontal asymptote, and (2) whether the function crosses its horizontal asymptote.

  1. \(f(x)=\displaystyle{\frac {x+3}{5x-2}}\)

  2. \(g(x)=\displaystyle{\frac {x^{2}+2x-1}{x-1}}\)

  3. \(h(x)=\displaystyle{\frac {x+1}{x^{2}+2x-1}}\)

Hint
Solution

Subsection0.6.4Summary

  • A polynomial of degree \(n\) has at most \(n\) real zeros and \(n-1\) turning points.

  • The degree of a polynomial function determines the end behavior of its graph. If the degree of a polynomial is even, then the end behavior is the same in both directions. If the degree of a polynomial is odd, then the end behavior on the left is the opposite of the behavior on the right.

  • A rational function is a function of the form \(f(x)=\frac{P(x)}{Q(x)}\text{,}\) where \(P(x)\) and \(Q(x)\) are both polynomials.

  • A rational function \(f(x)=\frac{P(x)}{Q(x)}\) may have a vertical asymptote whenever \(Q(x)=0\text{.}\)

  • The end behavior of the graph of a rational function is determined by the degrees of the polynomials in the numerator and denominator.

Subsubsection0.6.5Exercises

1

Without the aid of a graphing tool match the polynomials to its corresponding graph.

\begin{align*} f(x) \amp = x^3 + 3x^2 - 4x - 12\\ g(x) \amp = -\frac{1}{4}(x-1)^3(x+3)\\ h(x) \amp = \frac{1}{3}x^3(x+2)(x-3)^2\\ k(x) \amp = -2x^3-x^2+x \end{align*}

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2

The cost in dollars for removing \(p\) percent of pollutants from a river is

\begin{equation*} C(p) = \frac{61700p}{100-p}. \end{equation*}
  1. Find the cost for removing 20%.

  2. Find the cost for removing half of the pollutants.

  3. What is the smallest value \(p\) can be?

  4. What is the largest value \(p\) can be?

  5. What is the value of \(C(p)\) as \(p \to 100\text{?}\) What is the meaning of this in the context of the problem?

3

For the function

\begin{equation*} f(x) = \frac{2x-6}{(-6x-1)(6x-6)}, \end{equation*}
  1. what are the vertical asymptotes?

  2. what are the horizontal asymptotes?

  3. what are the coordinates of the \(x\) intercepts?

4

Square cuts of the same size are cut from a flat rectangular piece of cardboard. The remaining cardboard is folded into a lidless box. Assume that the cardboard originally measures 20 inches by 12 inches. Let \(x\) be the length of one side of the square cut (at this point you should stop and draw a picture).

  1. Write a function describing the volume of the resulting box in terms of \(x\text{.}\)

  2. What is an appropriate domain for the volume function? Plot the volume function over the domain.

  3. Write a function describing the surface area of the box in terms of \(x\text{.}\)

  4. Is the domain different for the surface area function? Why / why not? Plot the surface area function over its appropriate domain.