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2.2.2: Graphs of Polynomials Using Zeros

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    Graphs of Polynomials Using Zeros

    How is finding and using the zeroes of a higher-degree polynomial related to the same process you have used in the past on quadratic functions?

    Graphing Polynomials Using Zeros

    The following procedure can be followed when graphing a polynomial function.

    • Use the leading-term test to determine the end behavior of the graph.
    • Find the x−intercept(s) of f(x) by setting f(x)=0 and then solving for x.
    • Find the y−intercept of f(x) by setting y=f(0) and finding y.
    • Use the x−intercept(s) to divide the x−axis into intervals and then choose test points to determine the sign of f(x) on each interval.
    • Plot the test points.
    • If necessary, find additional points to determine the general shape of the graph.

    The Leading-Term Test

    If anxn is the leading term of a polynomial. Then the behavior of the graph as x→∞ or x→−∞ can be known by one the four following behaviors:

    2. If an<0 and n even: f-d_d7d5e0905838171941c98b5d7400249f875c025e564f361b2a215907+IMAGE_TINY+IMAGE_TINY.jpg

    3. If an>0 and n odd: f-d_192341c14f2284951a398f20e62b4c75b8af0317d93d383b5514b53e+IMAGE_TINY+IMAGE_TINY.jpg

    4. If an<0 and n odd: f-d_953895f7aa2a92e3fa4275f1973c297b95e8105a17f0791b0497ddef+IMAGE_TINY+IMAGE_TINY.jpg


    Example 1

    Earlier, you were asked to identify some similarities in graphing using zeroes between quadratic functions and higher-degree polynomials.


    Despite the more complex nature of the graphs of higher-degree polynomials, the general process of graphing using zeroes is actually very similar. In both cases, your goal is to locate the points where the graph crosses the x or y axis. In both cases, this is done by setting the y value equal to zero and solving for x to find the x axis intercepts, and setting the x value equal to zero and solving for y to find the y axis intercepts.

    Example 2

    Find the roots (zeroes) of the polynomial:



    Start by factoring:


    To find the zeros, set h(x)=0 and solve for x.


    This gives








    So we say that the solution set is {−3,−1,2}. They are the zeros of the function h(x). The zeros of h(x) are the x−intercepts of the graph y=h(x) below.

    Example 3

    Find the zeros of g(x)=−(x−2)(x−2)(x+1)(x+5)(x+5)(x+5).


    The polynomial can be written as


    To solve the equation, we simply set it equal to zero


    this gives








    Notice the occurrence of the zeros in the function. The factor (x−2) occurred twice (because it was squared), the factor (x+1) occurred once and the factor (x+5) occurred three times. We say that the zero we obtain from the factor (x−2) has a multiplicity k=2 and the factor (x+5) has a multiplicity k=3.

    Example 4

    Graph the polynomial function f(x)=−3x4+2x3.


    Since the leading term here is −3x4 then an=−3<0, and n=4 even. Thus the end behavior of the graph as x→∞ and x→−∞ is that of Box #2, item 2.

    We can find the zeros of the function by simply setting f(x)=0 and then solving for x.



    This gives

    x=0 or x=\(\ 2\over 3\)

    So we have two x−intercepts, at x=0 and at x=\(\ 2\over 3\), with multiplicity k=3 for x=0 and multiplicity k=1 for x=\(\ 2\over 3\)

    To find the y−intercept, we find f(0), which gives


    So the graph passes the y−axis at y=0.

    Since the x−intercepts are 0 and \(\ 2\over 3\), they divide the x−axis into three intervals: (−∞, 0), (0, \(\ 2\over 3\)), and (\(\ 2\over 3\), ∞). Now we are interested in determining at which intervals the function f(x) is negative and at which intervals it is positive. To do so, we construct a table and choose a test value for x from each interval and find the corresponding f(x) at that value.

    Interval Test Value x f(x) Sign of f(x) Location of points on the graph
    (−∞, 0) -1 -5 - below the x−axis
    (0, \(\ 2\over 3\)) \(\ 1\over 2\) \(\ 1\over 16\) + above the x−axis
    (\(\ 2\over 3\), ∞) 1 -1 - below the x−axis

    Those test points give us three additional points to plot: (−1, −5), (\(\ 1\over 2\), \(\ 1\over 16\)), and (1, -1). Now we are ready to plot our graph. We have a total of three intercept points, in addition to the three test points. We also know how the graph is behaving as x→−∞ and x→+∞. This information is usually enough to make a rough sketch of the graph. If we need additional points, we can simply select more points to complete the graph.

    Example 5

    Find the zeros and sketch a graph of the polynomial



    This is a factorable equation,



    Setting f(x)=0,


    the first term gives



    x= ±\(\ \sqrt{8}\)

    = ±\(\ 2\sqrt{2}\)

    and the second term gives



    x=±\(\ \sqrt{-7}\)

    =±\(\ i\sqrt{7}\)

    So the solutions are ±\(\ 2\sqrt{2}\) and ±\(\ i\sqrt{7}\), a total of four zeros of f(x). Keep in mind that only the real zeros of a function correspond to the x−intercept of its graph.

    Example 6

    Graph g(x)=−(x−2)2(x+1)(x+5)3.


    Use the zeros to create a table of intervals and see whether the function is above or below the x−axis in each interval:

    Interval Test value x g(x) Sign of g(x) Location of graph relative to x−axis
    (−∞, −5) -6 320 + Above
    x=−5 -5 0 NA  
    (-5, -1) -2 144 + Above
    x=−1 -1 0 NA  
    (-1, 2) 0 -100 - Below
    x=2 2 0 NA  
    (2, ∞) 3 -256 - Below

    Finally, use this information and the test points to sketch a graph of g(x).



    1. If c is a zero of f, then c is a/an _________________________ of the graph of f.
    2. If c is a zero of f, then (x - c) is a factor of ___________________?
    3. Find the zeros of the polynomial: P(x)=x3−5x2+6x

    Consider the function: f(x)=−3(x−3)4(5x−2)(2x−1)3(4−x)2.

    1. How many zeros (x-intercepts) are there?
    2. What is the leading term?

    Find the zeros and graph the polynomial. Be sure to label the x-intercepts, y-intercept (if possible) and have correct end behavior. You may use technology for questions 9-12.

    1. P(x)=−2(x+1)2(x−3)
    2. P(x)=x3+3x2−4x−12
    3. f(x)=−2x3+6x2+9x+6
    4. f(x)=−4x2−7x+3
    5. f(x)=2x5+4x3+8x2+6x
    6. f(x)=x4−3x2
    7. g(x)=x2−|x|
    8. Given: P(x)=(3x+2)(x−7)2(9x+2)3


    1. The leading term:
    2. The degree of the polynomial:
    3. The leading coefficient:

    Determine the equation of the polynomial based on the graph:

    1. f-d_a2c9f9878050e516e56a0eefb61d8313e702f6d50ab3d6df240aafad+IMAGE_TINY+IMAGE_TINY.png
    2. f-d_7e339675765d8aa3438a350c9e7e16eeadce092f4fd7b79435441ac0+IMAGE_TINY+IMAGE_TINY.png

    Review (Answers)

    To see the Review answers, open this PDF file and look for section 2.4.


    Term Definition
    Cubic Function A cubic function is a function containing an x3 term as the highest power of x.
    Intercept The intercepts of a curve are the locations where the curve intersects the x and y axes. An x intercept is a point at which the curve intersects the x-axis. A y intercept is a point at which the curve intersects the y-axis.
    interval An interval is a specific and limited part of a function.
    Leading-Term Test The leading-term test is a test to determine the end behavior of a polynomial function.
    Polynomial A polynomial is an expression with at least one algebraic term, but which does not indicate division by a variable or contain variables with fractional exponents.
    Polynomial Graph A polynomial graph is the graph of a polynomial function. The term is most commonly used for polynomial functions with a degree of at least three.
    Quartic Function A quartic function is a function f(x) containing an x4 term as the highest power of ''x''.
    Roots The roots of a function are the values of x that make y equal to zero.
    Zeroes The zeroes of a function f(x) are the values of x that cause f(x) to be equal to zero.

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