Precalculus Study Guide
Copyright Maplesoft, a division of Waterloo Maple Inc., 2024
Chapter 11: Functions Defined Implicitly
|
Introduction
|
|
Some equations of the form can be rewritten in the form , in which case, we say that the original equation defined implicitly, and the equation gives explicitly.
Some equations of the form cannot be rewritten in the form . For these, there is no combination of algebraic steps that will isolate the letter in the equation. However, such equations can still define in principle, even if the values have to be computed numerically. Such functions are said to be defined implicitly by the equation .
Some equations of the form do not define implicitly since they represent relations, one-many maps from the domain to the range. However, in such cases, we can define branches along which a function is defined implicitly by the equation .
This chapter explores examples of all three types of behaviors, the simple case such as posed by the equation
where can easily be isolated; the single-branch case such as posed by the equation
where cannot be isolated; and the multiple-branch case such as posed by
the familiar equation of the unit circle.
|
Chapter Glossary
|
|
The following terms in Chapter 11 are linked to the Maple Math Dictionary.
discretization
discriminant
domain
explicit
floating-point
|
implicit
inequality
interval
nonnegative
parabola
|
quadratic equation
quadratic formula
real number
sequence
square root
|
|
|
|
|
|
Typical Problems
|
|
11.1. The equation defines one or more functions implicitly.
(a) Obtain a graph of the implicitly defined functions.
(b) Express explicitly each function in the graph plotted in Part (a).
(c) Use the given equation at to compute all real values of .
11.2. The equation implicitly defines a function .
(a) On the interval , obtain (using some sort of plotting device) a graph of the implicitly defined function.
(b)
|
In the interval , substitute each node , into the given equation, then solve for the corresponding numerically. Plot the points and compare the graph to the one obtained in Part (a).
|
|
|
|
|
Maple Initializations
|
|
Before accessing the Maple version of the solutions to the typical problems stated above, initialize Maple by pressing the button provided on the right.
|
|
|
|
|
|
Solutions
|
|
|
Problem 11.1
|
|
|
11.1 - Mathematical Solution
|
|
|
11.1 (a) - Mathematical Solution
|
|
Obtaining a graph of the function defined implicitly by the equation
is a challenging task if done without the aid of an appropriate plotting device. One way to accomplish this by hand is to anticipate Part (b) and obtain explicit expressions for the two branches contained therein. Thus, write the equation in the form
and use the quadratic formula to obtain
|
|
|
|
|
|
The square roots appearing in each of these branches is real for . Again applying the quadratic formula and sketching the parabola represented by the left-hand side of this inequality, we discover that the domain for each of the two branches of the implicit function is
or approximately
Then, from the function values listed in Table 11.1.1, use a sheet of graph paper to sketch a graph akin to the one in Figure 11.1.1.
|
|
|
Table 11.1.1 Function values for branches and
|
Figure 11.1.1 Graph of implicitly-defined
|
|
|
|
|
11.1 (b) - Mathematical Solution
|
|
In Part (a), it was necessary to anticipate Part (b) where the two branches contained implicitly in the equation
are obtained explicitly. Recall that by use of the quadratic formula, applied to
the branches and were determined to be
|
|
|
|
|
|
|
|
11.1 (c) - Mathematical Solution
|
|
If the substitution is made in the equation
the equation
= 0
results. This equation determines the two values of that correspond to . Of course, this equation can be solved by the quadratic formula, yielding
|
|
|
|
|
|
|
|
|
|
|
|
|
11.1 - Maplet Solution
|
|
|
11.1 (a) - Maplet Solution
|
|
A graph of the implicit functions defined by the equation
can be obtained with the
Implicit Function Tutor
.
Clicking this link will launch the tutor with the solution embedded as shown in the thumbnail image in Figure 11.1.2.
For the graph, simply enter the defining equation and click the button labeled Graph. Use the Plot Options button as needed.
To launch the Implicit Function Tutor, click the following link:
Implicit Function Tutor
|
|
Figure 11.1.2 Thumbnail image of the Implicit Function Tutor
|
|
|
|
|
|
|
|
11.1 (b) - Maplet Solution
|
|
The explicit rules for the functions defined implicitly by the equation
can be obtained with the
Implicit Function Tutor
.
Clicking this link will launch the tutor with the solution embedded as shown in the thumbnail image Figure 11.1.2.
After entering the defining equation, click the button labeled Explicit Branches. The resulting expressions are then
|
|
Figure 11.1.2 Thumbnail image of the Implicit Function Tutor
|
|
|
|
To launch the Implicit Function Tutor, click the following link:
Implicit Function Tutor
|
|
|
|
|
11.1 (c) - Maplet Solution
|
|
To use the given equation to compute all real values of , again use the
Implicit Function Tutor
.
Clicking this link will launch the tutor with the solution embedded as shown in the thumbnail image in Figure 11.1.3.
In the section Numeric study, enter 1 as the value of , and for the Interval containing corresponding , enter to 5, as suggested by the graph.
Click the button labeled Equation for y to see the equation that results from substituting into the original equation.
Click the button labeled Analytic Solution to obtain the two values or click the button labeled Numeric Solution to obtain the values of in floating-point (decimal) form.
|
|
Figure 11.1.3 Thumbnail image of the Implicit Function Tutor
|
|
|
|
To launch the Implicit Function Tutor, click the following link:
Implicit Function Tutor
|
|
|
|
|
|
11.1 - Interactive Solution
|
|
|
11.1 (a) - Interactive Solution
|
|
•
|
Type (or Control-drag) the equation.
|
•
|
Context Panel: Plots≻2-D Implicit Plot≻
|
|
|
|
|
|
|
11.1 (b) - Interactive Solution
|
|
•
|
Type (or Control-drag) the equation.
|
•
|
Context Panel: Solve≻Solve for Variable≻
|
|
|
|
|
|
|
11.1 (c) - Interactive Solution
|
|
•
|
Type (or Control-drag) the equation.
Press the Enter key.
|
•
|
Context Panel: Evaluate at a Point≻
|
|
|
|
|
|
|
|
11.1 - Programmatic Solution
|
|
|
11.1 (a) - Programmatic Solution
|
|
The function defined implicitly by the equation
>
|
|
can be plotted with Maple's implicitplot command found in the plots package. Its use leads to Figure 11.1.4.
>
|
|
|
Figure 11.1.4 Graph of the function defined implicitly by
|
|
|
|
|
11.1 (b) - Programmatic Solution
|
|
The equation
can be solved explicitly for . Maple's solve command gives
Thus, there are two branches, which we label as
>
|
|
These two explicit branches of the implicitly defined function are plotted in Figure 11.1.5, where appears in black, and appears in red.
>
|
|
|
Figure 11.1.5 Graph of branches (black) and (red)
|
|
|
The algebra by which these two explicit branches are extracted from the equation
is no more than an application of the quadratic formula. The variable in the quadratic equation is , and all 's are treated as constants. Thus, the equation is written as
where , and , are respectively
>
|
|
The quadratic formula gives the two solutions for as
>
|
|
in agreement with Maple's earlier solution.
The discriminant of this quadratic in the variable is
which can also be obtained with Maple's discrim command, as shown by
>
|
|
This discriminant is nonnegative for in the interval
or, in floating-point form,
These calculations are consistent with Figure 11.1.5.
|
|
11.1 (c) - Programmatic Solution
|
|
To compute the value(s) of , substitute into the given equation, namely, into
obtaining
>
|
|
which is a quadratic equation in the variable . The solution can be obtained with the quadratic formula, or with Maple's solve command. In either event, the solutions will be
a result consistent with evaluating the branches
>
|
|
at . Indeed, we find
>
|
|
|
|
|
|
Problem 11.2
|
|
|
11.2 - Mathematical Solution
|
|
|
11.2 (a) - Mathematical Solution
|
|
Figure 11.2.1 contains a (Maple-generated) graph of the function defined implicitly by the equation
|
Figure 11.2.1 Graph of defined implicitly by the equation
|
|
|
|
|
11.2 (b) - Mathematical Solution
|
|
At a node , the equation
becomes one of the entries in Table 11.2.1.
|
Equation
|
|
Equation
|
0
|
|
11
|
|
1
|
|
12
|
|
2
|
|
13
|
|
3
|
|
14
|
|
4
|
|
15
|
|
5
|
|
16
|
|
6
|
|
17
|
|
7
|
|
18
|
|
8
|
|
19
|
|
9
|
|
20
|
|
10
|
|
|
|
Table 11.2.1 The equation
at the nodes
|
|
|
The solutions of the equations in Table 11.2.1 appear in Table 11.2.2. Figure 11.2.2 contains a graph of the data points in Table 11.2.2.
|
|
|
Table 11.2.2 Solutions of equations in Table 11.2.1
|
|
|
|
|
Figure 11.2.2 The curve from Figure 11.2.1 and the points from Table 11.2.2
|
|
|
|
|
|
|
|
|
11.2 - Maplet Solution
|
|
|
11.2 (a) - Maplet Solution
|
|
The function defined implicitly by the equation
can be graphed on an interval such as by means of the
Implicit Function Tutor
.
Clicking this link will launch the tutor with the solution embedded as shown in the thumbnail image in Figure 11.2.3.
After entering the defining equation, use the Plot Option button to set the plotting interval.
Click the Graph button to obtain the required graph.
|
|
Figure 11.2.3 Thumbnail image of the Implicit Function Tutor
|
|
|
|
To launch the Implicit Function Tutor, click the following link:
Implicit Function Tutor
|
|
|
|
|
11.2 (b) - Maplet Solution
|
|
The function defined implicitly by the equation
can be graphed pointwise on an interval such as by means of the
Implicit Function Tutor
.
Clicking this link will launch the tutor with the solution embedded as shown in the thumbnail image in Figure 12.2.4.
For each of the nodes , enter a single value of and click the button labeled Numeric Solution to obtain the corresponding value of .
|
|
Figure 11.2.4 Thumbnail image of the Implicit Function Tutor
|
|
|
|
One such calculation for is shown in the solution provided where the nonlinear equation defining this value of has also been obtained by clicking the button labeled Equation for .
After collecting the coordinates of all 21 points, plot the points, connecting them with a smooth curve. Of course, this task is tedious if executed by hand. For this problem, the sections Interactive Solution and Programmatic Solution implement more efficient strategies for these calculations.
To launch the Implicit Function Tutor, click the following link:
Implicit Function Tutor
|
|
|
|
|
|
11.2 - Interactive Solution
|
|
|
11.2 (a) - Interactive Solution
|
|
•
|
Type (or Control-drag) the equation, being sure to use the exponential "e" and not just the letter e.
|
•
|
Context Panel: Plots≻2-D Implicit Plot≻
|
|
|
|
|
|
|
11.2 (b) - Interactive Solution
|
|
Write the left-hand side of the equation as a function
|
•
|
Type (or Control-drag) the equation, being sure to use the exponential "e" and not just the letter e.
|
•
|
Context Panel: Left-hand Side
|
•
|
Context Panel: Conversions≻Operator≻
|
•
|
Context Panel: Assign to a Name≻
|
|
|
Generate a list of -values
|
•
|
Type and press the Enter key.
|
•
|
Context Panel: Sequence≻≻
|
•
|
Context Panel: Map Command Onto
Map the command: fsolve
Append the arguments:
|
•
|
Context Panel: Conversions≻To List
|
•
|
Context Panel: Assign to a Name≻
|
|
|
Generate corresponding list of -values
|
•
|
Type and press the Enter key.
|
•
|
Context Panel: Sequence≻≻
|
•
|
Context Panel: Conversions≻To List
|
•
|
Context Panel: Assign to a Name≻
|
|
|
Graph computed points
|
•
|
Type and press the Enter key.
|
•
|
Context Panel: Plots≻Plot Builder≻Plot
|
|
|
Graph implicit function
|
•
|
Type (or Control-drag) the equation, being sure to use the exponential "e" and not just the letter e.
|
•
|
Context Panel: Plots≻Plot Builder
|
|
|
Control-drag (or copy/paste) the graph of the implicit function onto the graph of the computed points.
|
|
|
|
|
|
11.2 - Programmatic Solution
|
|
|
11.2 (a) - Programmatic Solution
|
|
The function defined implicitly by the equation
>
|
|
can be plotted with Maple's implicitplot
command from the plots package. The result is seen in Figure 11.2.5.
>
|
|
|
Figure 11.2.5 Graph of defined implicitly by the equation
|
|
|
|
|
11.2 (b) - Programmatic Solution
|
|
The suggested discretization contains the nodes and . At the node , the equation
becomes
>
|
|
This equation in can be solved numerically, and for this, we use Maple's fsolve
command. The result is
Thus, one point on the graph of the implicitly defined function is
At the other end of the interval , we have , and the calculations at that node are
>
|
|
and
>
|
|
Thus, the point
is also on the graph of the implicitly defined function .
A more efficient process uses a Maple for-loop that results in a list of points from which Maple's plot
command can fashion a graph. This loop is implemented as follows.
>
|
|
The NULL command at the beginning of the calculations starts an empty sequence of points. As each new -value is computed, the point is formed and appended to the list.
A plot in which the computed points are linearly connected is then found in Figure 11.2.6.
>
|
|
|
Figure 11.2.4 Graph of and the discrete points in Table 11.2.2
|
|
|
|
|
|
|
|
Exercises - Chapter 11
|
|
The equations given in Exercises 11.1 - 11.5 define one or more functions implicitly. In each case,
(a) Using an appropriate technology, obtain a graph of the implicitly defined functions.
(b) Express explicitly each function in the graph plotted in Part (a).
(c) Use the given equation at to compute all real values of .
11.1.
11.2. , with in the interval .
11.3. , with in the interval .
11.4.
11.5.
|
Go to Chapter 1 2 3 4 5 6 7 8 9 10
Go to Contents
|