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3.2: Polynomial Functions and Their Graphs

3.2: Polynomial Functions and Their Graphs. Definition of a Polynomial Function Let n be a nonnegative integer and let a n , a n -1 ,…, a 2 , a 1 , a 0 , be real numbers with a n  0. The function defined by f ( x ) = a n x n + a n-1 x n -1 + … + a 2 x 2 + a 1 x + a 0

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3.2: Polynomial Functions and Their Graphs

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  1. 3.2: Polynomial Functions and Their Graphs Definition of a Polynomial Function Let n be a nonnegative integer and let an, an-1,…, a2, a1, a0, be real numbers with an 0. The function defined by f (x)=anxn+ an-1xn-1+…+a2x2+ a1x + a0 is called a polynomial function of xof degree n. The number an, the coefficient of the variable to the highest power, is called the leading coefficient.

  2. End Behavior of Polynomial Functions The Leading Coefficient Test As xincreases or decreases without bound, the graph of the polynomial function f (x)=anxn+ an-1xn-1+ an-2xn-2 +…+a1x + a0 (an 0) eventually rises or falls. In particular, 1. For n odd: an> 0an< 0 If the leading coefficient is positive, the graph falls to the left and rises to the right. If the leading coefficient is negative, the graph rises to the left and falls to the right. Rises right Rises left Falls right Falls left 3.2: Polynomial Functions and Their Graphs

  3. End Behavior of Polynomial Functions The Leading Coefficient Test As xincreases or decreases without bound, the graph of the polynomial function f (x)=anxn+ an-1xn-1+ an-2xn-2 +…+a1x + a0 (an 0) eventually rises or falls. In particular, 1. For n even: an> 0an< 0 If the leading coefficient is positive, the graph rises to the left and to the right. If the leading coefficient is negative, the graph falls to the left and to the right. Rises right Rises left Falls left Falls right 3.2: Polynomial Functions and Their Graphs

  4. Smooth, Continuous Graphs Two important features of the graphs of polynomial functions are that they are smooth and continuous. By smooth, we mean that the graph contains only rounded curves with no y y sharp corners. By continuous, we mean that the graph has no breaks and can be drawn without lifting your pencil from the rectangular coordinate system. These ideas are illustrated in the figure. Smooth rounded corner Smooth rounded corner x x Smooth rounded corner Smooth rounded corner 3.2: Polynomial Functions and Their Graphs

  5. Graphing a Polynomial Function 3.2: Polynomial Functions and Their Graphs f(x)=anxn+ an-1xn-1+ an-2xn-2 +  +a1x + a0 (an 0) 1. Use the Leading Coefficient Test to determine the graph's end behavior. 2. Find x-intercepts by settingf(x) = 0 and solving the resulting polynomial equation. If there is an x-intercept at r as a result of (x-r)k in the complete factorization of f(x), then: a. If k is even, the graph touches the x-axis at r and turns around. b. If k is odd, the graph crosses the x-axis at r. c. If k > 1, the graph flattens out at (r, 0). 3.Find the y-intercept by setting x equal to 0 and computing f(0).

  6. f(x)=anxn+ an-1xn-1+ an-2xn-2 +  +a1x + a0 (an 0) 3.2: Polynomial Functions and Their Graphs Graphing a Polynomial Function 3.Find the y-intercept by setting x equal to 0 and computing f(0). 4. Use symmetry, if applicable, to help draw the graph: a.y-axis symmetry: f(-x) =f(x) b. Origin symmetry: f(-x) =-f(x). 5. Use the fact that the maximum number of turning points of the graph is n - 1 to check whether it is drawn correctly.

  7. EXAMPLE:Using the Leading Coefficient Test Rises right y x Falls left 3.2: Polynomial Functions and Their Graphs Use the Leading Coefficient Test to determine the end behavior of the graph of Graph the quadratic functionf(x)= x3+3x2-x- 3. SolutionBecause the degree is odd (n=3) and the leading coefficient, 1, is positive, the graph falls to the left and rises to the right, as shown in the figure.

  8. 3.2: Polynomial Functions and Their Graphs EXAMPLE:Finding Zeros of a Polynomial Function Find all zeros of f(x)=-x4+4x3- 4x2. SolutionWe find the zeros of fby setting f(x)equal to 0. -x4+4x3- 4x2 = 0 We now have a polynomial equation. x4-4x3+ 4x2= 0 Multiply both sides by -1. (optional step) x2(x2- 4x+ 4)2= 0 Factor out x2. x2(x- 2)2= 0 Factor completely. x2= 0 or (x- 2)2= 0 Set each factor equal to zero. x= 0 x= 2Solve for x.

  9. y x 3.2: Polynomial Functions and Their Graphs EXAMPLE:Finding Zeros of a Polynomial Function Find all zeros of f(x)=-x4+4x3- 4x2. SolutionThe zeros of f(x)=-x4+4x3- 4x2 are 0 and 2. The graph of fhas x-intercepts at 0 and 2 as shown in the figure.We find the zeros of fby setting f(x)equal to 0.

  10. 3.2: Polynomial Functions and Their Graphs Multiplicity and x-Intercepts If r is a zero of even multiplicity, then the graph touches the x-axis and turns around at r. If r is a zero of odd multiplicity, then the graph crosses the x-axis at r. Regardless of whether a zero is even or odd, graphs tend to flatten out at zeros with multiplicity greater than one.

  11. Turning Points of Polynomial Functions The graph of f(x) = x5-6x3 + 8x + 1 is shown in the figure below. The graph has four smooth turning points. At each turning point, the graph changes direction from increasing to decreasing or vice versa. In calculus, these points are called local maxima or local minima. The given equation has 5 as its greatest exponent and is therefore a polynomial function of degree 5. Notice that the graph has four turning points. In general, if f isa polynomial ofdegree n, then the graph offhas at most n- 1 turning points. Turning points: from increasing to decreasing. y x Turning points: from decreasing to increasing. 3.2: Polynomial Functions and Their Graphs

  12. EXAMPLE: Graphing a PolynomialFunction Solution Step 1 Determine end behavior. Because the degree is even (n = 4) and the leading coefficient, 1, is positive, the graph rises to the left and the right: y Rises left Rises right x 3.2: Polynomial Functions and Their Graphs Graph: f(x)=x4-2x2+ 1.

  13. 3.2: Polynomial Functions and Their Graphs EXAMPLE: Graphing a PolynomialFunction EXAMPLE: Graphing a PolynomialFunction Graph: f(x)=x4-2x2+ 1. Solution Step 2 Find the x-intercepts (zeros of the function) by settingf(x)=0. x4-2x2+ 1 = 0 (x2- 1)(x2- 1) = 0 Factor. (x+ 1)(x -1)(x+ 1)(x -1) = 0 Factor completely. (x+ 1)2(x- 1)2= 0 Express the factoring in more compact notation. (x+ 1)2= 0 or (x- 1)2= 0 Set each factor equal to zero. x=-1 x= 1Solve for x.

  14. y Rises right Rises left x 1 1 3.2: Polynomial Functions and Their Graphs EXAMPLE: Graphing a PolynomialFunction Graph: f(x)=x4-2x2+ 1. Solution Step 2We see that -1 and 1 are both repeated zeros with multiplicity 2. Because of the even multiplicity, the graph touches the x-axis at -1 and 1 and turns around. Furthermore, the graph tends to flatten out at these zeros with multiplicity greater than one:

  15. y Rises right Rises left 1 1 1 x 3.2: Polynomial Functions and Their Graphs EXAMPLE: Graphing a PolynomialFunction Graph: f(x)=x4-2x2+ 1. Solution Step 3 Find the y-intercept. Replace xwith 0 in f(x)=-x+ 4x - 1. f(0) =04- 2 • 02+ 1=1 There is a y‑intercept at 1, so the graph passes through (0, 1).

  16. Replace xwith -x. 3.2: Polynomial Functions and Their Graphs EXAMPLE: Graphing a PolynomialFunction Graph: f(x)=x4-2x2+ 1. Solution Step 4 Use possible symmetry to help draw the graph. Our partial graph suggests y-axis symmetry. Let's verify this by finding f(-x). f(x) =x4-2x2+ 1 f(-x)= (-x)4-2(-x)2+ 1 =x4-2x2+ 1 Because f(-x)= f(x), the graph of f is symmetric with respect to the y-axis. The following figure shows the graph.

  17. y x 3.2: Polynomial Functions and Their Graphs EXAMPLE: Graphing a PolynomialFunction Graph: f(x)=x4-2x2+ 1. Solution Step 5 Use the fact that the maximum number of turning points of the graph is n- 1 to check whether it is drawn correctly. Because n= 4, the maximum number of turning points is 4 - 1, or 3. Because our graph has three turning points, we have not violated the maximum number possible.

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