What Is the Equation of an Ellipse?

Ellipse with focal point and axes

Ellipse with focal point and axes

Theory

The Ellipse

An ellipse consists of all points that have the same total distance to two given points, called focal points, or foci.

The following distances are important to describe an ellipse:

Semi-major axis (a):

The greatest distance from the center of the ellipse to any point on the ellipse.

Semi-minor axis (b):

The shortest distance from the center of the ellipse to any point on the ellipse.

Rule

The Standard Equation of an Ellipse

If both the focal points of an ellipse lie on the x-axis equidistant to the origin, and you call the semi-major axis a and the semi-minor axis b, the standard equation for the ellipse is

x2 a2 + y2 b2 = 1

A circle is a special case of an ellipse, where the focal points coincide and the semi-major and semi-minor axes are equal.

Example 1

Find the standard equation of an ellipse with a semi-major axis of a = 5 and a semi-minor axis of b = 3

x2 52 + y2 32 = 1 x2 25 + y2 9 = 1

Example 2

Show that 9x2 + 4y2 36 = 0 is the equation of an ellipse

To show that this equation represents an ellipse, you need to change it into the form of the standard equation of an ellipse. You can do that like this:

9x2 + 4y2 36 = 0 9x2 + 4y2 = 36 | ÷ 9 x2 + 4y2 9 = 4 | ÷ 4 x2 4 + y2 9 = 1

Because you’ve been able to change the expression into the form of the standard equation of an ellipse, you’ve shown that 9x2 + 4y2 36 = 0 is an ellipse with semi-minor axis a = 4 = 2 and semi-major axis b = 9 = 3.

Want to know more?Sign UpIt's free!