Precalculus Study Guide
Copyright Maplesoft, a division of Waterloo Maple Inc., 2024
Chapter 11: Functions Defined Implicitly
Introduction
Some equations of the form f⁡x,y=0 can be rewritten in the form y=g⁡x, in which case, we say that the original equation defined y⁡x implicitly, and the equation y=g⁡x gives y⁡x explicitly.
Some equations of the form f⁡x,y=0 cannot be rewritten in the form y=g⁡x. For these, there is no combination of algebraic steps that will isolate the letter y in the equation. However, such equations can still define y=g⁡x in principle, even if the values have to be computed numerically. Such functions y⁡x are said to be defined implicitly by the equation f⁡x,y=0.
Some equations of the form f⁡x,y=0 do not define y⁡x 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 y⁡x is defined implicitly by the equation f⁡x,y=0.
This chapter explores examples of all three types of behaviors, the simple case such as posed by the equation
2⁢x+3⁢y−7=0
where y can easily be isolated; the single-branch case such as posed by the equation
ln⁡y+x+y−5=0
where y cannot be isolated; and the multiple-branch case such as posed by
x2+y2=1
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 5⁢x2+3⁢x⁢y+2⁢y2−4⁢x−7⁢y−9=0 defines one or more functions y=y⁡x 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 x=1 to compute all real values of y⁡1.
11.2. The equation ln⁡x⁢e−y+x+y−5⁢x⁢y+3⁢x+7⁢y+9=0 implicitly defines a function y=y⁡x.
(a) On the interval 1,5, obtain (using some sort of plotting device) a graph of the implicitly defined function.
(b)
In the interval 2,5, substitute each node xk=2+⁡320⁢k,k=0,1,...,20, into the given equation, then solve for the corresponding y⁡xk numerically. Plot the points ⁡xk,y⁡xk 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
5⁢x2+3⁢x⁢y+2⁢y2−4⁢x−7⁢y−9=0
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
2⁢y2+3⁢x−7⁢y+⁡5⁢x2−4⁢x−9=0
and use the quadratic formula to obtain
y±
=−3⁢x−7±3⁢x−72−4⁢⁡2⁢5⁢x2−4⁢x−92⁢2
=147−3 x±121−10⁢x−31⁢x2
The square roots appearing in each of these branches is real for 121−10⁢x−31⁢x2≥0. 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
−5+8⁢5931≤x≤−5+8⁢5931
or approximately
−2.14≤x≤1.82
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.
xy+⁡xy−⁡x−2.04.282.22−1.84.651.55−1.64.851.05−1.44.950.65−1.25.000.30−1.05.000.00−0.84.96−0.26−0.64.89−0.49−0.44.79−0.69−0.24.66−0.86
xy+⁡xy−⁡x0.04.50−1.000.24.31−1.110.44.10−1.200.63.85−1.250.83.56−1.261.03.24−1.241.22.86−1.161.42.40−1.001.61.82−0.721.80.800.00
Table 11.1.1 Function values for branches y+ and y−
Figure 11.1.1 Graph of implicitly-defined y⁡x
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 y+ and y− were determined to be
11.1 (c) - Mathematical Solution
If the substitution x=1 is made in the equation
the equation
2⁢y2−4⁢y−8 = 0
results. This equation determines the two values of y that correspond to x=1. Of course, this equation can be solved by the quadratic formula, yielding
=−−4±−42−4⁢⁡2⁢⁡−82⁢2
=4 ±804
=1 ±5
={3.236067977−1.236067977
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
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
y±=14⁢7−3 x ±121−10⁢x−31⁢x2
11.1 (c) - Maplet Solution
To use the given equation to compute all real values of y⁡1, 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 x, and for the Interval containing corresponding y, enter y=−2 to 5, as suggested by the graph.
Click the button labeled Equation for y to see the equation that results from substituting x=1 into the original equation.
Click the button labeled Analytic Solution to obtain the two values y=1 ±5 or click the button labeled Numeric Solution to obtain the values of y in floating-point (decimal) form.
Figure 11.1.3 Thumbnail image of the 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≻x,y
11.1 (b) - Interactive Solution
Context Panel: Solve≻Solve for Variable≻y
11.1 (c) - Interactive Solution
Type (or Control-drag) the equation. Press the Enter key.
Context Panel: Evaluate at a Point≻x=1
Context Panel: Solve
11.1 - Programmatic Solution
11.1 (a) - Programmatic Solution
The function defined implicitly by the equation
q ≔ 5 x2 + 3 x y + 2 y2 − 4 x − 7 y − 9 = 0
can be plotted with Maple's implicitplot command found in the plots package. Its use leads to Figure 11.1.4.
implicitplot⁡q,x=−3..2,y=−2..5,scaling=constrained
Figure 11.1.4 Graph of the function defined implicitly by 5⁢x2+3⁢x⁢y+2⁢y2−4⁢x−7⁢y−9=0
11.1 (b) - Programmatic Solution
The equation
q
can be solved explicitly for y=y⁡x. Maple's solve command gives
Y ≔ solveq, y
Thus, there are two branches, which we label as
y+= Y1; y−= Y2
These two explicit branches of the implicitly defined function are plotted in Figure 11.1.5, where y+ appears in black, and y− appears in red.
plot⁡Y,x=−2.2..2,color=black,red,numpoints=2000,scaling=constrained,legend=typesety+,typesety−
Figure 11.1.5 Graph of branches y+ (black) and y− (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 y, and all x's are treated as constants. Thus, the equation is written as
A⁢y2+B⁢y+C=0
where A,B, and C, are respectively
A ≔ coefflhsq, y, 2;B ≔ coefflhsq, y, 1;C ≔ coefflhsq, y, 0
The quadratic formula gives the two solutions for y as
−B+sqrtB2−4 A C/2/A;−B−sqrtB2−4 A C/2/A
in agreement with Maple's earlier solution.
The discriminant of this quadratic in the variable y is
simplifyB2 − 4 A C
which can also be obtained with Maple's discrim command, as shown by
d ≔ discrimlhsq,y
This discriminant is nonnegative for x in the interval
r ≔ solved≥0, x
or, in floating-point form,
evalfr
These calculations are consistent with Figure 11.1.5.
11.1 (c) - Programmatic Solution
To compute the value(s) of y⁡1, substitute x=1 into the given equation, namely, into
obtaining
q1 ≔ evalq, x=1
which is a quadratic equation in the variable y. The solution can be obtained with the quadratic formula, or with Maple's solve command. In either event, the solutions will be
solveq1, y
a result consistent with evaluating the branches
y1 = Y1;y2 = Y2
at x=1. Indeed, we find
simplifyevalY1, x=1;simplifyevalY2, x=1
Problem 11.2
11.2 - Mathematical Solution
11.2 (a) - Mathematical Solution
Figure 11.2.1 contains a (Maple-generated) graph of the function y⁡x defined implicitly by the equation
ln⁡x⁢e−y+x+y−5⁢x⁢y+3⁢x+7⁢y+9=0
Figure 11.2.1 Graph of y⁡x defined implicitly by the equationln⁡x⁢e−y+x+y−5⁢x⁢y+3⁢x+7⁢y+9=0
11.2 (b) - Mathematical Solution
At a node xk=2+⁡320⁢k,k=0,1,...,20, the equation
becomes one of the entries in Table 11.2.1.
k
Equation
0
ln⁡2−4⁢y+2+y+15=0
11
ln⁡7320+39920−494⁢y+110⁢365+100⁢y=0
1
ln⁡4320+30920−194⁢y+110⁢215+100⁢y=0
12
ln⁡195−13⁢y+15⁢95+25⁢y+1025=0
2
ln⁡2310−112⁢y+110⁢230+100⁢y+15910=0
13
ln⁡7920+41720−554⁢y+110⁢395+100⁢y=0
3
ln⁡4920+32720−254⁢y+110⁢245+100⁢y=0
14
ln⁡4110−292⁢y+110⁢410+100⁢y+21310=0
4
ln⁡135−7⁢y+15⁢65+25⁢y+845=0
15
ln⁡174+874−614⁢y+12⁢17+4⁢y=0
5
ln⁡114+694−314⁢y+12⁢11+4⁢y=0
16
ln⁡225+1115−16⁢y+15⁢110+25⁢y=0
6
ln⁡2910−172⁢y+110⁢290+100⁢y+17710=0
17
ln⁡9120+45320−674⁢y+110⁢455+100⁢y=0
7
ln⁡6120+36320−374⁢y+110⁢305+100⁢y=0
18
ln⁡4710−352⁢y+110⁢470+100⁢y+23110=0
8
ln⁡165−10⁢y+15⁢80+25⁢y+935=0
19
ln⁡9720+47120−734⁢y+110⁢485+100⁢y=0
9
ln⁡6720+38120−434⁢y+110⁢335+100⁢y=0
20
ln⁡5−19⁢y+5+y+24=0
10
ln⁡72−232⁢y+12⁢14+4⁢y+392=0
Table 11.2.1 The equation ln⁡x⁢e−y+x+y−5⁢x⁢y+3⁢x+7⁢y+9=0
at the nodes xk=2+⁡320⁢k,k=0,1,...,20
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.
xy⁡x2.004.562.153.932.303.482.453.142.602.872.752.662.902.48
xy⁡x3.052.333.202.213.352.103.502.013.651.933.801.853.951.79
xy⁡x4.101.734.251.684.401.634.551.594.701.554.851.525.001.48
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 y⁡x defined implicitly by the equation
ln⁡x⁢ⅇ−y+x+y−5⁢x⁢y+3⁢x+7⁢y+9=0
can be graphed on an interval such as 1,5 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
11.2 (b) - Maplet Solution
can be graphed pointwise on an interval such as 2,5 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 xk=2+⁡320⁢k,k=0,1,...,20, enter a single value of x and click the button labeled Numeric Solution to obtain the corresponding value of y.
Figure 11.2.4 Thumbnail image of the Implicit Function Tutor
One such calculation for x=2 is shown in the solution provided where the nonlinear equation defining this value of y has also been obtained by clicking the button labeled Equation for y.
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.
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.
11.2 (b) - Interactive Solution
Write the left-hand side of the equation as a function fx
Press the Enter key.
Context Panel: Left-hand Side
Context Panel: Conversions≻Operator≻x
Context Panel: Assign to a Name≻f
Generate a list of y-values
Type f2+320k=0 and press the Enter key.
Context Panel: Sequence≻k≻0..20
Context Panel: Map Command Onto Map the command: fsolve Append the arguments: y
Context Panel: Conversions≻To List
Context Panel: Assign to a Name≻Y
Generate corresponding list of x-values
Type 2+320k and press the Enter key.
Context Panel: Assign to a Name≻X
Graph computed points
Type X,Y and press the Enter key.
Context Panel: Plots≻Plot Builder≻Plot
Graph implicit function
Context Panel: Plots≻Plot Builder 2≤x≤5
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
q ≔ lnx exp−y + sqrtx+y − 5 x y + 3 x + 7 y + 9 = 0
can be plotted with Maple's implicitplot command from the plots package. The result is seen in Figure 11.2.5.
implicitplot⁡q,x=1..5,y=1..5
Figure 11.2.5 Graph of y⁡x defined implicitly by the equationln⁡x⁢e−y+x+y−5⁢x⁢y+3⁢x+7⁢y+9=0
11.2 (b) - Programmatic Solution
The suggested discretization contains the nodes x0=2 and x20=5. At the node x2, the equation
becomes
q0 ≔ evalq, x=2
This equation in y can be solved numerically, and for this, we use Maple's fsolve command. The result is
y0 ≔ fsolveq0, y
Thus, one point on the graph of the implicitly defined function y=y⁡x is
``2,y0
At the other end of the interval 2,5, we have x20=5, and the calculations at that node are
q20 ≔ evalq, x=5
and
y20 ≔ fsolveq20, y
Thus, the point
``5,y20
is also on the graph of the implicitly defined function y=y⁡x.
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.
P ≔ NULL:for k from 0 to 20 do X ≔ 2 + 3/20 k;Y ≔ fsolveevalq,x=X,y;P ≔ P,X,Y; end do: P
The NULL command at the beginning of the calculations starts an empty sequence of points. As each new y-value is computed, the point⁡xk,yk 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.
p1:=plot⁡P,color=red: p2:=plot⁡P,style=point,symbol=solidcircle,color=black: display⁡p1,p2,labels=x,y
Figure 11.2.4 Graph of y⁡x 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 y=y⁡x implicitly. In each case,
(a) Using an appropriate technology, obtain a graph of the implicitly defined functions.
11.1. 9⁢x2+10⁢y2+10⁢x−2⁢y−6=0
11.2. 2⁢x2−9⁢y2+10⁢x−3⁢y+3=0, with x in the interval −10,10.
11.3. 2⁢x2−4⁢x⁢y+y2+2⁢x−12⁢y+5=0, with x in the interval −15,15.
11.4. 7⁢x2+5⁢x⁢y+11⁢y2+9⁢x−3⁢y−7=0
11.5. 5⁢x2+10⁢x⁢y+11⁢y2+10⁢x+y−50=0
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