If there are two foci then there are two

**focal radii**. Note: Using this second definition, the sum of the**focal radii**of an ellipse is a constant. The**difference**of the**focal radii**of a**hyperbola**is a constant. It is the distance between the vertices.In this way, what is the focus of a hyperbola?

Two fixed points located inside each curve of a

**hyperbola**that are used in the curve's formal definition. A**hyperbola**is defined as follows: For two given points, the**foci**, a**hyperbola**is the locus of points such that the difference between the distance to each**focus**is constant.Is foci and focus the same?

The word

**foci**(pronounced 'foe-sigh') is the plural of '**focus**'. One**focus**, two**foci**. If the major axis and minor axis are the**same**length, the figure is a circle and both**foci**are at the center. Reshape the ellipse above and try to create this situation.How do you find the foci?

Each ellipse has two

**foci**(plural of focus) as shown in the picture here: As you can see, c is the distance from the center to a focus. We can find the value of c by using the**formula**c^{2}= a^{2}- b^{2}. Notice that this**formula**has a negative sign, not a positive sign like the**formula**for a hyperbola.