# Foci of an Ellipse

Written by Jerry Ratzlaff on . Posted in Plane Geometry

Foci is a point used to define the conic section.  F and G seperately are called "focus", both togeather are called "foci".

### Foci of an Ellipse formula

$$\large{ c^2 = a^2 - b^2 }$$

Where:

$$\large{ c }$$ = length center to focus

$$\large{ a }$$ = length semi-major axis

$$\large{ b }$$ = length semi-minor axis

$$\large{ F }$$ and $$\large{ G }$$ = focus