Pre-Calculus Prep: VOLUME 6: Vectors
Pre-Calculus Prep: VOLUME 6: Vectors

Table of Contents

Learning Objectives

In this section, you will:

• Parameterize a curve.
• Eliminate the parameter.
• Find a rectangular equation for a curve defined parametrically.
• Find parametric equations for curves defined by rectangular equations.

Consider the path a moon follows as it orbits a planet, which simultaneously rotates around the sun, as seen in Figure 1. At any moment, the moon is located at a particular spot relative to the planet. But how do we write and solve the equation for the position of the moon when the distance from the planet, the speed of the moon’s orbit around the planet, and the speed of rotation around the sun are all unknowns? We can solve only for one variable at a time.

In this section, we will consider sets of equations given by and where is the independent variable of time. We can use these parametric equations in a number of applications when we are looking for not only a particular position but also the direction of the movement. As we trace out successive values of the orientation of the curve becomes clear. This is one of the primary advantages of using parametric equations: we are able to trace the movement of an object along a path according to time. We begin this section with a look at the basic components of parametric equations and what it means to parameterize a curve. Then we will learn how to eliminate the parameter, translate the equations of a curve defined parametrically into rectangular equations, and find the parametric equations for curves defined by rectangular equations.

Parameterizing a Curve

When an object moves along a curve—or curvilinear path—in a given direction and in a given amount of time, the position of the object in the plane is given by the x-coordinate and the y-coordinate. However, both and vary over time and so are functions of time. For this reason, we add another variable, the parameter, upon which both and are dependent functions. In the example in the section opener, the parameter is time, The position of the moon at time, is represented as the function and the position of the moon at time, is represented as the function Together, and are called parametric equations, and generate an ordered pair Parametric equations primarily describe motion and direction.

When we parameterize a curve, we are translating a single equation in two variables, such as and into an equivalent pair of equations in three variables, and One of the reasons we parameterize a curve is because the parametric equations yield more information: specifically, the direction of the object’s motion over time.

When we graph parametric equations, we can observe the individual behaviors of and of There are a number of shapes that cannot be represented in the form meaning that they are not functions. For example, consider the graph of a circle, given as Solving for gives or two equations: and If we graph and together, the graph will not pass the vertical line test, as shown in Figure 2. Thus, the equation for the graph of a circle is not a function.

However, if we were to graph each equation on its own, each one would pass the vertical line test and therefore would represent a function. In some instances, the concept of breaking up the equation for a circle into two functions is similar to the concept of creating parametric equations, as we use two functions to produce a non-function. This will become clearer as we move forward.

Parametric Equations

Suppose is a number on an interval, The set of ordered pairs, where and forms a plane curve based on the parameter The equations and are the parametric equations.

Example 1

Parameterizing a Curve

Parameterize the curve letting Graph both equations.

Solution

If then to find we replace the variable with the expression given in In other words, Make a table of values similar to Table 1, and sketch the graph.

See the graphs in Figure 3. It may be helpful to use the TRACE feature of a graphing calculator to see how the points are generated as increases.

Analysis

The arrows indicate the direction in which the curve is generated. Notice the curve is identical to the curve of

Try It #1

Construct a table of values and plot the parametric equations:

Example 2

Finding a Pair of Parametric Equations

Find a pair of parametric equations that models the graph of using the parameter Plot some points and sketch the graph.

Solution

If and we substitute for into the equation, then Our pair of parametric equations is

To graph the equations, first we construct a table of values like that in Table 2. We can choose values around from to The values in the column will be the same as those in the column because Calculate values for the column

The graph of is a parabola facing downward, as shown in Figure 4. We have mapped the curve over the interval shown as a solid line with arrows indicating the orientation of the curve according to Orientation refers to the path traced along the curve in terms of increasing values of As this parabola is symmetric with respect to the line the values of are reflected across the y-axis.

Try It #2

Parameterize the curve given by

Example 3

Finding Parametric Equations That Model Given Criteria

An object travels at a steady rate along a straight path to in the same plane in four seconds. The coordinates are measured in meters. Find parametric equations for the position of the object.

Solution

The parametric equations are simple linear expressions, but we need to view this problem in a step-by-step fashion. The x-value of the object starts at meters and goes to 3 meters. This means the distance x has changed by 8 meters in 4 seconds, which is a rate of or We can write the x-coordinate as a linear function with respect to time as In the linear function template and

Similarly, the y-value of the object starts at 3 and goes to which is a change in the distance y of −4 meters in 4 seconds, which is a rate of or We can also write the y-coordinate as the linear function Together, these are the parametric equations for the position of the object, where and are expressed in meters and represents time:

Using these equations, we can build a table of values for and (see Table 3). In this example, we limited values of to non-negative numbers. In general, any value of can be used.

From this table, we can create three graphs, as shown in Figure 5.

Analysis

Again, we see that, in Figure 5(c), when the parameter represents time, we can indicate the movement of the object along the path with arrows.

Eliminating the Parameter

In many cases, we may have a pair of parametric equations but find that it is simpler to draw a curve if the equation involves only two variables, such as and Eliminating the parameter is a method that may make graphing some curves easier. However, if we are concerned with the mapping of the equation according to time, then it will be necessary to indicate the orientation of the curve as well. There are various methods for eliminating the parameter from a set of parametric equations; not every method works for every type of equation. Here we will review the methods for the most common types of equations.

Eliminating the Parameter from Polynomial, Exponential, and Logarithmic Equations

For polynomial, exponential, or logarithmic equations expressed as two parametric equations, we choose the equation that is most easily manipulated and solve for We substitute the resulting expression for into the second equation. This gives one equation in and

Example 4

Eliminating the Parameter in Polynomials

Given and eliminate the parameter, and write the parametric equations as a Cartesian equation.

Solution

We will begin with the equation for because the linear equation is easier to solve for

Next, substitute for in

The Cartesian form is

Analysis

This is an equation for a parabola in which, in rectangular terms, is dependent on From the curve’s vertex at the graph sweeps out to the right. See Figure 6. In this section, we consider sets of equations given by the functions and where is the independent variable of time. Notice, both and are functions of time; so in general is not a function of

Try It #3

Given the equations below, eliminate the parameter and write as a rectangular equation for as a function of

Example 5

Eliminating the Parameter in Exponential Equations

Eliminate the parameter and write as a Cartesian equation: and

Solution

Isolate

Substitute the expression into

The Cartesian form is

Example 6

Eliminating the Parameter in Logarithmic Equations

Eliminate the parameter and write as a Cartesian equation: and

Solution

Solve the first equation for

Then, substitute the expression for into the equation.

The Cartesian form is

Analysis

To be sure that the parametric equations are equivalent to the Cartesian equation, check the domains. The parametric equations restrict the domain on to we restrict the domain on to The domain for the parametric equation is restricted to we limit the domain on to

Try It #4

Eliminate the parameter and write as a rectangular equation.

Eliminating the Parameter from Trigonometric Equations

Eliminating the parameter from trigonometric equations is a straightforward substitution. We can use a few of the familiar trigonometric identities and the Pythagorean Theorem.

First, we use the identities:

Solving for and we have

Then, use the Pythagorean Theorem:

Substituting gives

Example 7

Eliminating the Parameter from a Pair of Trigonometric Parametric Equations

Eliminate the parameter from the given pair of trigonometric equations where and sketch the graph.

Solution

Solving for and we have

Next, use the Pythagorean identity and make the substitutions.

The graph for the equation is shown in Figure 8.

Analysis

Applying the general equations for conic sections (introduced in Analytic Geometry, we can identify as an ellipse centered at Notice that when the coordinates are and when the coordinates are This shows the orientation of the curve with increasing values of

Try It #5

Eliminate the parameter from the given pair of parametric equations and write as a Cartesian equation: and

Finding Cartesian Equations from Curves Defined Parametrically

When we are given a set of parametric equations and need to find an equivalent Cartesian equation, we are essentially “eliminating the parameter.” However, there are various methods we can use to rewrite a set of parametric equations as a Cartesian equation. The simplest method is to set one equation equal to the parameter, such as In this case, can be any expression. For example, consider the following pair of equations.

Rewriting this set of parametric equations is a matter of substituting for Thus, the Cartesian equation is

Example 8

Finding a Cartesian Equation Using Alternate Methods

Use two different methods to find the Cartesian equation equivalent to the given set of parametric equations.

Solution

Method 1. First, let’s solve the equation for Then we can substitute the result into the equation.

Now substitute the expression for into the equation.

Method 2. Solve the equation for and substitute this expression in the equation.

Make the substitution and then solve for

Try It #6

Write the given parametric equations as a Cartesian equation: and

Finding Parametric Equations for Curves Defined by Rectangular Equations

Although we have just shown that there is only one way to interpret a set of parametric equations as a rectangular equation, there are multiple ways to interpret a rectangular equation as a set of parametric equations. Any strategy we may use to find the parametric equations is valid if it produces equivalency. In other words, if we choose an expression to represent and then substitute it into the equation, and it produces the same graph over the same domain as the rectangular equation, then the set of parametric equations is valid. If the domain becomes restricted in the set of parametric equations, and the function does not allow the same values for as the domain of the rectangular equation, then the graphs will be different.

Example 9

Finding a Set of Parametric Equations for Curves Defined by Rectangular Equations

Find a set of equivalent parametric equations for

Solution

An obvious choice would be to let Then But let’s try something more interesting. What if we let Then we have

The set of parametric equations is

See Figure 9.

Media

Access these online resources for additional instruction and practice with parametric equations.

8.6 Section Exercises

Verbal

What is a system of parametric equations?

Some examples of a third parameter are time, length, speed, and scale. Explain when time is used as a parameter.

Explain how to eliminate a parameter given a set of parametric equations.

What is a benefit of writing a system of parametric equations as a Cartesian equation?

What is a benefit of using parametric equations?

Why are there many sets of parametric equations to represent on Cartesian function?

Algebraic

For the following exercises, eliminate the parameter to rewrite the parametric equation as a Cartesian equation.

For the following exercises, rewrite the parametric equation as a Cartesian equation by building an table.

For the following exercises, parameterize (write parametric equations for) each Cartesian equation by setting or by setting

For the following exercises, parameterize (write parametric equations for) each Cartesian equation by using and Identify the curve.

Parameterize the line from to so that the line is at at and at at

Parameterize the line from to so that the line is at at and at at

Parameterize the line from to so that the line is at at and at at

Parameterize the line from to so that the line is at at and at at

Technology

For the following exercises, use the table feature in the graphing calculator to determine whether the graphs intersect.

For the following exercises, use a graphing calculator to complete the table of values for each set of parametric equations.

 –1 0 1
 1 2 3
 -1 0 1 2

Extensions

Find two different sets of parametric equations for

Find two different sets of parametric equations for

Find two different sets of parametric equations for

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