## Section 7.4: Parametric Equations

### Learning Outcomes

• 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. Figure 1

In this section, we will consider sets of equations given by $x\left(t\right)$ and $y\left(t\right)$ where $t$ 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 $t$, 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 $x$ and $y$
vary over time and so are functions of time. For this reason, we add another variable, the parameter, upon which both $x$ and $y$ are dependent functions. In the example in the section opener, the parameter is time, $t$. The $x$ position of the moon at time, $t$, is represented as the function $x\left(t\right)$, and the $y$ position of the moon at time, $t$, is represented as the function $y\left(t\right)$. Together, $x\left(t\right)$ and $y\left(t\right)$ are called parametric equations, and generate an ordered pair $\left(x\left(t\right),y\left(t\right)\right)$. Parametric equations primarily describe motion and direction.

When we parameterize a curve, we are translating a single equation in two variables, such as $x$ and $y$, into an equivalent pair of equations in three variables, $x,y$, and $t$. 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 $x$ and of $y$. There are a number of shapes that cannot be represented in the form $y=f\left(x\right)$, meaning that they are not functions. For example, consider the graph of a circle, given as ${r}^{2}={x}^{2}+{y}^{2}$. Solving for $y$ gives $y=\pm \sqrt{{r}^{2}-{x}^{2}}$, or two equations: ${y}_{1}=\sqrt{{r}^{2}-{x}^{2}}$ and ${y}_{2}=-\sqrt{{r}^{2}-{x}^{2}}$. If we graph ${y}_{1}$ and ${y}_{2}$ 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. Figure 2

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.

### A General Note: Parametric Equations

Suppose $t$ is a number on an interval, $I$. The set of ordered pairs, $\left(x\left(t\right),y\left(t\right)\right)$, where $x=f\left(t\right)$ and $y=g\left(t\right)$, forms a plane curve based on the parameter $t$. The equations $x=f\left(t\right)$ and $y=g\left(t\right)$ are the parametric equations.

### Example 1: Parameterizing a Curve

Parameterize the curve $y={x}^{2}-1$ letting $x\left(t\right)=t$. Graph both equations.

### Try It

Construct a table of values and plot the parametric equations: $x\left(t\right)=t - 3,y\left(t\right)=2t+4;-1\le t\le 2$.

### Example 2: Finding a Pair of Parametric Equations

Find a pair of parametric equations that models the graph of $y=1-{x}^{2}$, using the parameter $x\left(t\right)=t$. Plot some points and sketch the graph.

### Try It

Parameterize the curve given by $x={y}^{3}-2y$.

### Example 3: Finding Parametric Equations That Model Given Criteria

An object travels at a steady rate along a straight path $\left(-5,3\right)$ to $\left(3,-1\right)$ in the same plane in four seconds. The coordinates are measured in meters. Find parametric equations for the position of the object.

## Methods for Finding Cartesian and Polar Equations from Curves

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 $x$ and $y$. 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 $t$ 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 $t$. We substitute the resulting expression for $t$ into the second equation. This gives one equation in $x$ and $y$.

### Example 4: Eliminating the Parameter in Polynomials

Given $x\left(t\right)={t}^{2}+1$ and $y\left(t\right)=2+t$, eliminate the parameter, and write the parametric equations as a Cartesian equation.

### Try It

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

\begin{align}&x\left(t\right)=2{t}^{2}+6 \\ &y\left(t\right)=5-t\end{align}

### Example 5: Eliminating the Parameter in Exponential Equations

Eliminate the parameter and write as a Cartesian equation: $x\left(t\right)={e}^{-t}$ and $y\left(t\right)=3{e}^{t},t>0$.

### Example 6: Eliminating the Parameter in Logarithmic Equations

Eliminate the parameter and write as a Cartesian equation: $x\left(t\right)=\sqrt{t}+2$ and $y\left(t\right)=\mathrm{log}\left(t\right)$.

### Try It

Eliminate the parameter and write as a rectangular equation.

\begin{align}&x\left(t\right)={t}^{2} \\ &y\left(t\right)=\mathrm{ln}t,t>0\end{align}

## 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:

$\begin{gathered}x\left(t\right)=a\cos t\\ y\left(t\right)=b\sin t\end{gathered}$

Solving for $\cos t$ and $\sin t$, we have

$\begin{gathered}\frac{x}{a}=\cos t\\ \frac{y}{b}=\sin t\end{gathered}$

Then, use the Pythagorean Theorem:

${\cos }^{2}t+{\sin }^{2}t=1$

Substituting gives

${\cos }^{2}t+{\sin }^{2}t={\left(\frac{x}{a}\right)}^{2}+{\left(\frac{y}{b}\right)}^{2}=1$

### Example 7: Eliminating the Parameter from a Pair of Trigonometric Parametric Equations

Eliminate the parameter from the given pair of trigonometric equations where $0\le t\le 2\pi$ and sketch the graph.

\begin{align}&x\left(t\right)=4\cos t\\ &y\left(t\right)=3\sin t\end{align}

### Try It

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

$x\left(t\right)=2\cos t$ and $y\left(t\right)=3\sin t$.

### Try It

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 $x\left(t\right)=t$. In this case, $y\left(t\right)$ can be any expression. For example, consider the following pair of equations.

\begin{align}&x\left(t\right)=t\\ &y\left(t\right)={t}^{2}-3\end{align}

Rewriting this set of parametric equations is a matter of substituting $x$ for $t$. Thus, the Cartesian equation is $y={x}^{2}-3$.

### 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.

\begin{align}&x\left(t\right)=3t - 2 \\ &y\left(t\right)=t+1 \end{align}

### Try It

Write the given parametric equations as a Cartesian equation: $x\left(t\right)={t}^{3}$ and $y\left(t\right)={t}^{6}$.

## 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 $x$, and then substitute it into the $y$ 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 $x$ as the domain of the rectangular equation, then the graphs will be different.

The following video shows examples of how to find Cartesian representations of parametric equations of different kinds.

### Example 9: Finding a Set of Parametric Equations for Curves Defined by Rectangular Equations

Find a set of equivalent parametric equations for $y={\left(x+3\right)}^{2}+1$.

## Key Concepts

• Parameterizing a curve involves translating a rectangular equation in two variables, $x$ and $y$, into two equations in three variables, x, y, and t. Often, more information is obtained from a set of parametric equations.
• Sometimes equations are simpler to graph when written in rectangular form. By eliminating $t$, an equation in $x$ and $y$ is the result.
• To eliminate $t$, solve one of the equations for $t$, and substitute the expression into the second equation.
• Finding the rectangular equation for a curve defined parametrically is basically the same as eliminating the parameter. Solve for $t$ in one of the equations, and substitute the expression into the second equation.
• There are an infinite number of ways to choose a set of parametric equations for a curve defined as a rectangular equation.
• Find an expression for $x$ such that the domain of the set of parametric equations remains the same as the original rectangular equation.

## Glossary

parameter
a variable, often representing time, upon which $x$ and $y$ are both dependent

## Section 7.4 Homework Exercises

1. What is a system of parametric equations?

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

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

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

5. What is a benefit of using parametric equations?

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

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

7. $\begin{cases}x\left(t\right)=5-t\hfill \\ y\left(t\right)=8 - 2t\hfill \end{cases}$

8. $\begin{cases}x\left(t\right)=6 - 3t\hfill \\ y\left(t\right)=10-t\hfill \end{cases}$

9. $\begin{cases}x\left(t\right)=2t+1\hfill \\ y\left(t\right)=3\sqrt{t}\hfill \end{cases}$

10. $\begin{cases}x\left(t\right)=3t - 1\hfill \\ y\left(t\right)=2{t}^{2}\hfill \end{cases}$

11. $\begin{cases}x\left(t\right)=2{e}^{t}\hfill \\ y\left(t\right)=1 - 5t\hfill \end{cases}$

12. $\begin{cases}x\left(t\right)={e}^{-2t}\hfill \\ y\left(t\right)=2{e}^{-t}\hfill \end{cases}$

13. $\begin{cases}x\left(t\right)=4\text{log}\left(t\right)\hfill \\ y\left(t\right)=3+2t\hfill \end{cases}$

14. $\begin{cases}x\left(t\right)=\text{log}\left(2t\right)\hfill \\ y\left(t\right)=\sqrt{t - 1}\hfill \end{cases}$

15. $\begin{cases}x\left(t\right)={t}^{3}-t\hfill \\ y\left(t\right)=2t\hfill \end{cases}$

16. $\begin{cases}x\left(t\right)=t-{t}^{4}\hfill \\ y\left(t\right)=t+2\hfill \end{cases}$

17. $\begin{cases}x\left(t\right)={e}^{2t}\hfill \\ y\left(t\right)={e}^{6t}\hfill \end{cases}$

18. $\begin{cases}x\left(t\right)={t}^{5}\hfill \\ y\left(t\right)={t}^{10}\hfill \end{cases}$

19. $\begin{cases}x\left(t\right)=4\text{cos}t\hfill \\ y\left(t\right)=5\sin t \hfill \end{cases}$

20. $\begin{cases}x\left(t\right)=3\sin t\hfill \\ y\left(t\right)=6\cos t\hfill \end{cases}$

21. $\begin{cases}x\left(t\right)=2{\text{cos}}^{2}t\hfill \\ y\left(t\right)=-\sin t \hfill \end{cases}$

22. $\begin{cases}x\left(t\right)=\cos t+4\\ y\left(t\right)=2{\sin }^{2}t\end{cases}$

23. $\begin{cases}x\left(t\right)=t - 1\\ y\left(t\right)={t}^{2}\end{cases}$

24. $\begin{cases}x\left(t\right)=-t\\ y\left(t\right)={t}^{3}+1\end{cases}$

25. $\begin{cases}x\left(t\right)=2t - 1\\ y\left(t\right)={t}^{3}-2\end{cases}$

For the following exercises, rewrite the parametric equation as a Cartesian equation by building an $x\text{-}y$ table.

26. $\begin{cases}x\left(t\right)=2t - 1\\ y\left(t\right)=t+4\end{cases}$

27. $\begin{cases}x\left(t\right)=4-t\\ y\left(t\right)=3t+2\end{cases}$

28. $\begin{cases}x\left(t\right)=2t - 1\\ y\left(t\right)=5t\end{cases}$

29. $\begin{cases}x\left(t\right)=4t - 1\\ y\left(t\right)=4t+2\end{cases}$

For the following exercises, parameterize (write parametric equations for) each Cartesian equation by setting $x\left(t\right)=t$ or by setting $y\left(t\right)=t$.

30. $y\left(x\right)=3{x}^{2}+3$

31. $y\left(x\right)=2\sin x+1$

32. $x\left(y\right)=3\mathrm{log}\left(y\right)+y$

33. $x\left(y\right)=\sqrt{y}+2y$

For the following exercises, parameterize (write parametric equations for) each Cartesian equation by using $x\left(t\right)=a\cos t$ and $y\left(t\right)=b\sin t$. Identify the curve.

34. $\frac{{x}^{2}}{4}+\frac{{y}^{2}}{9}=1$

35. $\frac{{x}^{2}}{16}+\frac{{y}^{2}}{36}=1$

36. ${x}^{2}+{y}^{2}=16$

37. ${x}^{2}+{y}^{2}=10$

38. Parameterize the line from $\left(3,0\right)$ to $\left(-2,-5\right)$ so that the line is at $\left(3,0\right)$ at $t=0$, and at $\left(-2,-5\right)$ at $t=1$.

39. Parameterize the line from $\left(-1,0\right)$ to $\left(3,-2\right)$ so that the line is at $\left(-1,0\right)$ at $t=0$, and at $\left(3,-2\right)$ at $t=1$.

40. Parameterize the line from $\left(-1,5\right)$ to $\left(2,3\right)$ so that the line is at $\left(-1,5\right)$ at $t=0$, and at $\left(2,3\right)$ at $t=1$.

41. Parameterize the line from $\left(4,1\right)$ to $\left(6,-2\right)$ so that the line is at $\left(4,1\right)$ at $t=0$, and at $\left(6,-2\right)$ at $t=1$.

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

42. $\begin{cases}{x}_{1}\left(t\right)=3t\hfill \\ {y}_{1}\left(t\right)=2t - 1\hfill \end{cases}\text{ and }\begin{cases}{x}_{2}\left(t\right)=t+3\hfill \\ {y}_{2}\left(t\right)=4t - 4\hfill \end{cases}$

43. $\begin{cases}{x}_{1}\left(t\right)={t}^{2}\hfill \\ {y}_{1}\left(t\right)=2t - 1\hfill \end{cases}\text{ and }\begin{cases}{x}_{2}\left(t\right)=-t+6\hfill \\ {y}_{2}\left(t\right)=t+1\hfill \end{cases}$

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

44. $\begin{cases}{x}_{1}\left(t\right)=3{t}^{2}-3t+7\hfill \\ {y}_{1}\left(t\right)=2t+3\hfill \end{cases}$

$t$ $x$ $y$
–1
0
1

45. $\begin{cases}{x}_{1}\left(t\right)={t}^{2}-4\hfill \\ {y}_{1}\left(t\right)=2{t}^{2}-1\hfill \end{cases}$

$t$ $x$ $y$
1
2
3

46. $\begin{cases}{x}_{1}\left(t\right)={t}^{4}\hfill \\ {y}_{1}\left(t\right)={t}^{3}+4\hfill \end{cases}$

$t$ $x$ $y$
-1
0
1
2

47. Find two different sets of parametric equations for $y={\left(x+1\right)}^{2}$.

48. Find two different sets of parametric equations for $y=3x - 2$.

49. Find two different sets of parametric equations for $y={x}^{2}-4x+4$.