
Writing Equations of Ellipses Not Centered at the Origin
Like the graphs of other equations, the graph of an ellipse can be translated. If an ellipse is translated units horizontally and
units vertically, the center of the ellipse will be
. This translation results in the standard form of the equation we saw previously, with
replaced by
and
replaced by
.
Standard Forms of the Equation of an Ellipse with Center 
The standard form of the equation of an ellipse with center and major axis parallel to the
-axis is
Where
- the length of the major axis is
- the coordinates of the vertices are
- the length of the minor axis is
- the coordinates of the co-vertices are
- the coordinates of the foci are
, where
. See Figure 7 a
The standard form of the equation of an ellipse with center and major axis parallel to the
-axis is
Where
- the length of the major axis is
- the coordinates of the vertices are
- the length of the minor axis is
- the coordinates of the co-vertices are
- the coordinates of the foci are
, where
. See Figure 7b
Figure 7 (a) Horizontal ellipse with center (b) Vertical ellipse with center
HOW TO
Given the vertices and foci of an ellipse not centered at the origin, write its equation in standard form.
-
Determine whether the major axis is parallel to the
- or
-axis.
-
Identify the center of the ellipse
using the midpoint formula and the given coordinates for the vertices.
-
Find
by solving for the length of the major axis,
, which is the distance between the given vertices.
-
Find
using
and
, found in Step 2, along with the given coordinates for the foci.
- Substitute the values for
, and
into the standard form of the equation determined in Step 1.
Example 2
Writing the Equation of an Ellipse Centered at a Point Other Than the Origin
What is the standard form equation of the ellipse that has vertices and
and foci
and
?
Solution
The -coordinates of the vertices and foci are the same, so the major axis is parallel to the
-axis. Thus, the equation of the ellipse will have the form
First, we identify the center, . The center is halfway between the vertices,
and
. Applying the midpoint formula, we have:
Next, we find . The length of the major axis,
, is bounded by the vertices. We solve for
by finding the distance between the
-coordinates of the vertices.
Now we find . The foci are given by
. So,
and
. We substitute
using either of these points to solve for
.
Next, we solve for using the equation
.
Finally, we substitute the values found for , and
into the standard form equation for an ellipse: