1. 9.4 Graph & Write Equations of Ellipses
Algebra II

2. An ellipse is a set of points such that the distance between that point and two fixed points called Foci remains constant d1 d2 f1 f2 d4 d3
d1 + d2 = d3 + d4

3. cv1 F1 F2 v1 c v2
cv2

4. The line that goes through the Foci is the Major Axis.
The midpoint of that segment between the foci is the Center of the ellipse (c)
The intersection of the major axis and the ellipse itself results in two points, the Vertices (v)
The line that passes through the center and is perpendicular to the major axis is called the Minor Axis
The intersection of the minor axis and the ellipse results in two points known as co-vertices

5. Example of ellipse with vertical major axis

6. Example of ellipse with horizontal major axis

7. Standard Form for Elliptical Equations
Major Axis
(length is 2a)
Minor Axis (length is 2b)
Vertices
Co-Vertices
Horizontal
Vertical
(a,0) (-a,0)
(0,b) (0,-b)
(0,a) (0,-a)
(b,0) (-b,0)
Note that a is the biggest number!!!

8. The foci lie on the major axis at the points:
(c,0) (-c,0) for horizontal major axis
(0,c) (0,-c) for vertical major axis
Where c2 = a2 – b2

9. 1.) Write the equation of an ellipse with center (0,0) that has a vertex at (0,7) & co-vertex at (-3,0)
Since the vertex is on the y axis (0,7) a=7
The co-vertex is on the x-axis (-3,0) b=3
The ellipse has a vertical major axis & is of the form

10. Ex. 2) Given the equation 9x2 + 16y2 = 144 Identify:foci, vertices, & co-vertices
First put the equation in standard form:

11. From this we know the major axis is horizontal & a=4, b=3
So the vertices are (4,0) & (-4,0)
the co-vertices are (0,3) & (0,-3)
To find the foci we use c2 = a2 – b2
c2 = 16 – 9
c = √7
So the foci are at (√7,0) (-√7,0)

12. Graph Ex. 2

13. Ex. 3 Write an equation of the ellipse given a vertex & a focus & a center at (0,0)
Co-vertex: (0,2)

14. Standard Form for Elliptical Equations
Major Axis
(length is 2a)
Minor Axis (length is 2b)
Vertices
Co-Vertices
Horizontal
Vertical
(a,0) (-a,0)
(0,b) (0,-b)
(0,a) (0,-a)
(b,0) (-b,0)
Note that a is the biggest number!!!

15. Ex. 4 Write an equation of the ellipse given a vertex & a focus & a center at (0,0)

16. Ex. 5 Write an equation of the ellipse given a vertex & a focus & a center at (0,0)
Co-vertex:
Focus: (-15,0)

17. Assignment