Conics

# Hyperbolas

### Learning Objectives

By the end of this section, you will be able to:

• Graph a hyperbola with center at
• Graph a hyperbola with center at
• Identify conic sections by their equations

Before you get started, take this readiness quiz.

1. Solve:
If you missed this problem, review (Figure).
2. Expand:
If you missed this problem, review (Figure).
3. Graph
If you missed this problem, review (Figure).

### Graph a Hyperbola with Center at (0, 0)

The last conic section we will look at is called a hyperbola. We will see that the equation of a hyperbola looks the same as the equation of an ellipse, except it is a difference rather than a sum. While the equations of an ellipse and a hyperbola are very similar, their graphs are very different.

We define a hyperbola as all points in a plane where the difference of their distances from two fixed points is constant. Each of the fixed points is called a focus of the hyperbola.

Hyperbola

A hyperbola is all points in a plane where the difference of their distances from two fixed points is constant. Each of the fixed points is called a focus of the hyperbola.

The line through the foci, is called the transverse axis. The two points where the transverse axis intersects the hyperbola are each a vertex of the hyperbola. The midpoint of the segment joining the foci is called the center of the hyperbola. The line perpendicular to the transverse axis that passes through the center is called the conjugate axis. Each piece of the graph is called a branch of the hyperbola.

Again our goal is to connect the geometry of a conic with algebra. Placing the hyperbola on a rectangular coordinate system gives us that opportunity. In the figure, we placed the hyperbola so the foci are on the x-axis and the center is the origin.

The definition states the difference of the distance from the foci to a point is constant. So is a constant that we will call so We will use the distance formula to lead us to an algebraic formula for an ellipse.

To graph the hyperbola, it will be helpful to know about the intercepts. We will find the x-intercepts and y-intercepts using the formula.

The a, b values in the equation also help us find the asymptotes of the hyperbola. The asymptotes are intersecting straight lines that the branches of the graph approach but never intersect as the x, y values get larger and larger.

To find the asymptotes, we sketch a rectangle whose sides intersect the x-axis at the vertices and intersect the y-axis at The lines containing the diagonals of this rectangle are the asymptotes of the hyperbola. The rectangle and asymptotes are not part of the hyperbola, but they help us graph the hyperbola.

The asymptotes pass through the origin and we can evaluate their slope using the rectangle we sketched. They have equations and

There are two equations for hyperbolas, depending whether the transverse axis is vertical or horizontal. We can tell whether the transverse axis is horizontal by looking at the equation. When the equation is in standard form, if the x2-term is positive, the transverse axis is horizontal. When the equation is in standard form, if the y2-term is positive, the transverse axis is vertical.

The second equations could be derived similarly to what we have done. We will summarize the results here.

Standard Form of the Equation a Hyperbola with Center

The standard form of the equation of a hyperbola with center is

Notice that, unlike the equation of an ellipse, the denominator of is not always and the denominator of is not always

Notice that when the -term is positive, the transverse axis is on the x-axis. When the -term is positive, the transverse axis is on the y-axis.

Standard Forms of the Equation a Hyperbola with Center
Orientation Transverse axis on the x-axis.
Opens left and right
Transverse axis on the y-axis.
Opens up and down
Vertices
x-intercepts none
y-intercepts none
Rectangle Use Use
asymptotes

We will use these properties to graph hyperbolas.

How to Graph a Hyperbola with Center

Graph

Graph

Graph

We summarize the steps for reference.

Graph a hyperbola centered at
1. Write the equation in standard form.
2. Determine whether the transverse axis is horizontal or vertical.
3. Find the vertices.
4. Sketch the rectangle centered at the origin intersecting one axis at and the other at
5. Sketch the asymptotes—the lines through the diagonals of the rectangle.
6. Draw the two branches of the hyperbola.

Sometimes the equation for a hyperbola needs to be first placed in standard form before we graph it.

Graph

 To write the equation in standard form, divideeach term by 64 to make the equation equal to 1. Simplify. Since the y2-term is positive, the transverse axis is vertical.Since then The vertices are on the y-axis, Since then Sketch the rectangle intersecting the x-axis at and the y-axis at the vertices.Sketch the asymptotes through the diagonals of the rectangle.Draw the two branches of the hyperbola.

Graph

Graph

### Graph a Hyperbola with Center at

Hyperbolas are not always centered at the origin. When a hyperbola is centered at the equations changes a bit as reflected in the table.

Standard Forms of the Equation a Hyperbola with Center
Orientation Transverse axis is horizontal.
Opens left and right
Transverse axis is vertical.
Opens up and down
Center
Vertices a units to the left and right of the center a units above and below the center
Rectangle Use a units left/right of center
b units above/ below the center
Use a units above/below the center
b units left/right of center
How to Graph a Hyperbola with Center

Graph

Graph

Graph

We summarize the steps for easy reference.

Graph a hyperbola centered at
1. Write the equation in standard form.
2. Determine whether the transverse axis is horizontal or vertical.
3. Find the center and a, b.
4. Sketch the rectangle centered at using a, b.
5. Sketch the asymptotes—the lines through the diagonals of the rectangle. Mark the vertices.
6. Draw the two branches of the hyperbola.

Be careful as you identify the center. The standard equation has and with the center as

Graph

 Since the term is positive, the hyperbolaopens up and down. Find the center, Center: Find a, b. Sketch the rectangle that goes through thepoints 3 units above and below the center and2 units to the left/right of the center.Sketch the asymptotes—the lines through thediagonals of the rectangle.Mark the vertices.Graph the branches.

Graph

Graph

Again, sometimes we have to put the equation in standard form as our first step.

Write the equation in standard form and graph

 To get to standard form, complete the squares. Divide each term by 36 to get the constant to be 1. Since the term is positive, the hyperbolaopens left and right. Find the center, Center: Find a, b. Sketch the rectangle that goes through thepoints 3 units to the left/right of the centerand 2 units above and below the center.Sketch the asymptotes—the lines through thediagonals of the rectangle.Mark the vertices.Graph the branches.

Write the equation in standard form and graph

Write the equation in standard form and graph

### Identify Conic Sections by their Equations

Now that we have completed our study of the conic sections, we will take a look at the different equations and recognize some ways to identify a conic by its equation. When we are given an equation to graph, it is helpful to identify the conic so we know what next steps to take.

To identify a conic from its equation, it is easier if we put the variable terms on one side of the equation and the constants on the other.

Conic Characteristics of and terms Example
Parabola Either OR Only one variable is squared.
Circle and terms have the same coefficients
Ellipse and terms have the same sign, different coefficients
Hyperbola and terms have different signs, different coefficients

Identify the graph of each equation as a circle, parabola, ellipse, or hyperbola.

Identify the graph of each equation as a circle, parabola, ellipse, or hyperbola.

circle ellipse parabola hyperbola

Identify the graph of each equation as a circle, parabola, ellipse, or hyperbola.

ellipse parabola circle hyperbola

Access these online resources for additional instructions and practice with hyperbolas.

### Key Concepts

• Hyperbola: A hyperbola is all points in a plane where the difference of their distances from two fixed points is constant.

Each of the fixed points is called a focus of the hyperbola.
The line through the foci, is called the transverse axis.
The two points where the transverse axis intersects the hyperbola are each a vertex of the hyperbola.
The midpoint of the segment joining the foci is called the center of the hyperbola.
The line perpendicular to the transverse axis that passes through the center is called the conjugate axis.
Each piece of the graph is called a branch of the hyperbola.

Standard Forms of the Equation a Hyperbola with Center
Orientation Transverse axis on the x-axis.
Opens left and right
Transverse axis on the y-axis.
Opens up and down
Vertices
x-intercepts none
y-intercepts none ,
Rectangle Use Use
asymptotes
• How to graph a hyperbola centered at
1. Write the equation in standard form.
2. Determine whether the transverse axis is horizontal or vertical.
3. Find the vertices.
4. Sketch the rectangle centered at the origin intersecting one axis at and the other at
5. Sketch the asymptotes—the lines through the diagonals of the rectangle.
6. Draw the two branches of the hyperbola.

Standard Forms of the Equation a Hyperbola with Center
Orientation Transverse axis is horizontal.
Opens left and right
Transverse axis is vertical.
Opens up and down
Center
Vertices a units to the left and right of the center a units above and below the center
Rectangle Use a units left/right of center
b units above/below the center
Use a units above/below the center
b units left/right of center
• How to graph a hyperbola centered at
1. Write the equation in standard form.
2. Determine whether the transverse axis is horizontal or vertical.
3. Find the center and
4. Sketch the rectangle centered at using
5. Sketch the asymptotes—the lines through the diagonals of the rectangle. Mark the vertices.
6. Draw the two branches of the hyperbola.

Conic Characteristics of and terms Example
Parabola Either OR Only one variable is squared.
Circle and terms have the same coefficients
Ellipse and terms have the same sign, different coefficients
Hyperbola and terms have different signs, different coefficients

#### Practice Makes Perfect

Graph a Hyperbola with Center at

In the following exercises, graph.

Graph a Hyperbola with Center at

In the following exercises, graph.

In the following exercises, write the equation in standard form and graph.

Identify the Graph of each Equation as a Circle, Parabola, Ellipse, or Hyperbola

In the following exercises, identify the type of graph.

parabola circle hyperbola ellipse

Mixed Practice

In the following exercises, graph each equation.

#### Writing Exercises

In your own words, define a hyperbola and write the equation of a hyperbola centered at the origin in standard form. Draw a sketch of the hyperbola labeling the center, vertices, and asymptotes.

Explain in your own words how to create and use the rectangle that helps graph a hyperbola.

Compare and contrast the graphs of the equations and

Explain in your own words, how to distinguish the equation of an ellipse with the equation of a hyperbola.

#### Self Check

After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.

On a scale of 1-10, how would you rate your mastery of this section in light of your responses on the checklist? How can you improve this?

### Glossary

hyperbola
A hyperbola is defined as all points in a plane where the difference of their distances from two fixed points is constant.