Chapter 3: Applications of Differentiation
Section 3.7: What Derivatives Reveal about Graphs
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Example 3.7.2
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Graph , ; then use the tools of the calculus to analyze the features of this graph.
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Solution
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Initialize
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Tools≻Load Package: Student Calculus 1
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Loading Student:-Calculus1
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Control-drag
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Context Panel: Assign Function
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Curve Analysis Tutor
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Figure 3.7.2(a), an image of the
tutor, illustrates the features of the graph of that can be determined from itself, and from the derivatives and .
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Where is increasing or decreasing, its graph is drawn in red or black, respectively,
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Intervals where the graph of is concave up or down are shaded in gray or yellow, respectively.
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Relative extrema and inflection points are shown in green.
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Selecting one of the eight radio-buttons and clicking the "Calculate" button yields the information listed in Table 3.7.2(a).
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Figure 3.7.2(b) uses the FunctionChart (a.k.a. FunctionPlot) command to draw the graph contained in Figure 3.7.2(a). The command provides slightly more control over the features of the graph. The symbols for the seven green points can be made larger, and arrows are used to indicate concavity.
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The -intercepts are marked with circles; the inflection points, with crosses; and the extreme points with diamonds. These distinctions are not visible in the tutor.
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Figure 3.7.2(a) Curve Analysis tutor applied to
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>
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Student:-SetColors(red,black,green,gray,yellow):
Student:-Calculus1:-FunctionChart(x^3+5*x^2-17*x-9,x=-9..5,pointoptions=[symbolsize=20],caption=[],concavity=[filled(gray,yellow)]);
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Figure 3.7.2(b) Graph via the FunctionChart command
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Table 3.7.2(a) displays the information that would be provided by the "Calculate" button in the tutor.
The local maxima occur at:
[-4.57, 77.7]
[5., 156.]
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The local minima occur at:
[-9., -180.]
[1.24, -20.5]
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The function is increasing on the intervals:
[-9., -4.57]
[1.24, 5.]
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The function is decreasing on the interval:
[-4.57, 1.24]
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The function is concave up on the interval:
[-1.67, 5.]
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The function is concave down on the interval:
[-9., -1.67]
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The points of inflection occur at:
[-1.67, 28.7]
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The zeros occur at :
-7.19
-.470
2.66
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Table 3.7.2(a) Data generated by the Curve Analysis tutor for
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Graph the Function and its Derivatives
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Graph
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Figure 3.7.2(c) Graph of
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Figure 3.7.2(d) Graph of
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Figure 3.7.2(e) Graph of
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Obtain the Critical Numbers
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Obtain the critical numbers and by solving the equation
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Write and press the Enter key.
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Context Panel: Solve≻Numerically Solve
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Context Panel: Conversions≻To List
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Context Panel: Assign Name≻c
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The equation is quadratic, so there is an exact solution for the critical numbers. However, these numbers are binomial surds (radicals) that are hard to estimate without conversion to floating-point form.
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Second-Derivative Test
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Apply the Second-Derivative test
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Since is negative, the point is a relative maximum, a conclusion that is consistent with Figure 3.7.2(c).
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=
=
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Since is positive, the point is a relative minimum, a conclusion that is consistent with Figure 3.7.2(c).
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=
=
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Candidates for Inflection
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Obtain candidates for inflection points by solving the equation
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Write and press the Enter key.
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Context Panel: Solve≻Obtain Solutions for≻
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Context Panel: Assign Name≻
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Zeros of the Function
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Find the -intercepts by solving the equation
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Write and press the Enter key.
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Context Panel: Solve≻Numerically Solve
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Maple can provide the exact solution of a cubic equation. Unfortunately, it is the rare cubic for which the roots are simple expressions. Here, even after drastically simplifying the exact solutions, the expressions are unwieldy. Hence, a numeric solution is obtained instead.
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Conclusions
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The point = is a relative maximum. The point
= is also a relative maximum. From Figure 3.7.2(c), it is the absolute maximum.
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The point = is a relative minimum. The point = is also a relative minimum. From Figure 3.7.2(c), it is the absolute minimum.
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From Figures 3.7.9 and 3.7.10, the function increases on the intervals = and = . The function decreases on the interval = .
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From Figures 3.7.2(c) and 3.7.2(e), the function is concave upward on the interval = , and concave downward on the interval =
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From Figures 3.7.2(c) and 3.7.2(e), the point =
is an inflection point.
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The endpoints of a finite domain for the function have to be considered when searching for extrema. If the domain is unrestricted, that is, if it is the full set of real numbers for which the rule of the function is defined, then for this function, there would not be a global maximum or minimum because is unbounded as .
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Some Useful Commands
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Applicable Commands
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=
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=
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=
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=
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