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Graphing Polynomials

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Finding and Defining Parts of a Polynomial Function Graph

The prototype for a roller coaster is represented by the equation y = x^5 - 8x^3 + 10x + 6 . What is the maximum height the coaster will reach over the domain [-1, 2].

Watch This

First watch this video.

James Sousa: Ex: Increasing/ Decreasing/ Relative Extrema from Analyzing a Graph

Then watch this video.

James Sousa: Summary of End Behavior or Long Run Behavior of Polynomial Functions

Guidance

By now, you should be familiar with the general idea of what a polynomial function graph does. It should cross the x- axis as many times as the degree, unless there are imaginary solutions. It will curve up and down and can have a maximum and a minimum. Let’s define the parts of a polynomial function graph here.

Notice that in both the cubic (third degree, on the left) and the quartic (fourth degree, on the right) functions, there is no vertex. We now have minimums and maximums. If there are more than one minimum or maximum, there will be an absolute maximum/minimum , which is the lowest/highest point of the graph. A local maximum/minimum is a maximum/minimum relative to the points around it. The places where the function crosses the x- axis are still the solutions (also called x- intercepts, roots or zeros). In the quartic function, there is a repeated root at x = 4 . A repeated root will touch the x- axis without passing through or it can also have a “jump” in the curve at that point (see Example A). All of these points together (maximums, minimums, x- intercepts, and y- intercept) are called critical values.

Another important thing to note is end behavior. It is exactly what it sounds like; how the “ends” of the graph behaves or points. The cubic function above has ends that point in the opposite direction. We say that from left to right, this function is mostly increasing. The quartic function’s ends point in the same direction, both positive, just like a quadratic function. When considering end behavior, look at the leading coefficient and the degree of the polynomial.

Example A

Use a table to graph y=x^3 .

Solution: Draw a table and pick at least 5 values for x .

x x^3 y
-2 (-2)^3 -8
-1 (-1)^3 -1
0 0^3 0
1 1^3 1
2 2^3 8

Plot the points and connect. This particular function is the parent graph for cubic functions. Recall from quadratic functions, that the parent graph has a leading coefficient of 1, no other x- terms, and no y- intercept. y=x^4 and y=x^5 are also parent graphs.

Example B

Analyze the graph below. Find the critical values, end behavior, and find the domain and range.

Solution: First, find the solutions. They appear to be (-2, 0), (1, 0), and (2, 0). Therefore, this function has a minimum degree of 3. However, look at the y- intercept. The graph slightly bends between the maximum and minimum. This movement in the graph tells us that there are two imaginary solutions (recall that imaginary solutions always come in pairs). Therefore, the function has a degree of 5. Approximate the other critical values:

maximum: (-1.1, 10)

minimum: (1.5, -1.3)

y- intercept: (0, 5)

In general, this function is mostly increasing and the ends go in opposite directions. The domain and range are both all real numbers.

When describing critical values, you may approximate their location. In the next concept, we will use the graphing calculator to find these values exactly.

Sometime it can be tricky to see if a function has imaginary solutions from the graph. Compare the graph in Example B to the cubic function above. Notice that it is smooth between the maximum and minimum. As was pointed out earlier, the graph from Example B bends. Any function with imaginary solutions will have a slightly irregular shape or bend like this one does.

Example C

Sketch a graph of a function with roots -4, -3, \frac{1}{2} , and 3, has an absolute maximum at (2, 5), and has negative end behavior. This function does not have any imaginary roots.

Solution: There are several possible answers for this graph because we are only asking for a sketch. You would need more information to get an exact answer. Because this function has negative end behavior and four roots, we know that it will pass through the x- axis four times and face down. The absolute maximum is located between the roots \frac{1}{2} and 3. Plot these five points and connect to form a graph.

Intro Problem Revisit

Use a table to graph x^5 - 8x^3 + 10x + 6 .

Draw a table and pick at least 5 values for x . Remember that we are dealing only with x values between and including -1 and 2.

x y
-1 3
0 6
0.5 10.03125
1 9
2 -16

Plot the points and connect.

From your graph you can see that the maximum height the roller coaster reaches is just slightly over 10.

Guided Practice

1. Use a table to graph f(x)=-(x+2)^2(x-3) .

2. Analyze the graph. Find all the critical values, domain, range and describe the end behavior.

3. Draw a graph of the cubic function with solutions of -6 and a repeated root at 1. This function is generally increasing and has a maximum value of 9.

Answers

1. This function is in intercept form. Because the factor, (x + 2) is squared, we know it is a repeated root. Therefore, the function should just touch at -2 and not pass through the x- axis. There is also a zero at 3. Because the function is negative, it will be generally decreasing. Think of the slope of the line between the two endpoints. It would be negative. Select several points around the zeros to see the behavior of the graph.

x y
-4 14
-2 0
0 12
2 16
3 0
4 -36

2. There are three real zeros at approximately -3.5, 1, and 7. Notice the curve between the zeros 1 and 7. This indicated there are two imaginary zeros, making this at least a fifth-degree polynomial. Think about an imaginary horizontal line at y = 3 . This line would touch the graph five times, so there should be five solutions. Next, there is an absolute minimum at (-0.5, -7.5), a local maximum at (2.25, 5), a local minimum at (2.25, 2.25) and an absolute maximum at (5, 6). The y- intercept is at (0, -6). The domain and range are both all real numbers and the end behavior is mostly decreasing.

3. To say the function is “mostly increasing” means that the slope of the line that connects the two ends (arrows) is positive. Then, the function must pass through (-6, 0) and touch, but not pass through (1, 0). From this information, the maximum must occur between the two zeros and the minimum will be the double root.

Vocabulary

Absolute Maximum/Minimum
The highest/lowest point of a function. When referring to the absolute maximum/minimum value, use the y- value.
Local Maximum/Minimum
The highest/lowest point relative to the points around it. A function can have multiple local maximums or minimums.
Solutions
The x- intercepts. Also called roots or zeros.
Critical Values
The x- intercepts, maximums, minimums, and y- intercept.
End Behavior
How the ends of a graph look. End behavior depends on the degree of the function and the leading coefficient.
Parent Graph
The most basic function of a particular type. It has a leading coefficient of 1, no additional x- terms, and no constant.

Practice

Use the given x- values to make a table and graph the functions below.

  1. f(x) &= x^3-7x^2+15x-2\\x &= -2, -1, 0, 1, 2, 3, 4
  2. g(x) &= -2x^4 - 11x^3 - 3x^2+37x+35\\x &= -5, -4, -3, -2, -1, 0, 1, 2
  3. y &=2x^3+25x^2+100x+125\\x &= -7,-6,-5,-4,-3,-2,-1,0

Make your own table and graph the following functions.

  1. f(x)=(x+5)(x+2)(x-1)
  2. y=x^4
  3. y=x^5
  4. Analyze the graphs of y=x^2, y=x^3, y=x^4 , and y=x^5 . These are all parent functions. What do you think the graph of y=x^6 and y=x^7 will look like? What can you say about the end behavior of all even functions? Odd functions? What are the solutions to these functions?
  5. Writing How many repeated roots can one function have? Why?

Analyze the graphs of the following functions. Find all critical values, the domain, range, and end behavior.

For questions 13-15, make a sketch of the following real-solution functions.

  1. Draw two different graphs of a cubic function with zeros of -1, 1, and 4.5 and a minimum of -4.
  2. A fourth-degree polynomial with roots of -3.2, -0.9, 1.2, and 8.7, positive end behavior, and a local minimum of -1.7.
  3. A fourth-degree function with solutions of -7, -4, 1, and 2, negative end behavior, and an absolute maximum at \left(-\frac{11}{2}, \frac{1755}{128}\right) .
  4. Challenge Find the equation of the function from #15.

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