Parts of an Ellipse

Diagram of an ellipse

This diagram of a horizontal ellipse shows the ellipse itself in red, the center $C$ at the origin, the focal points at $\left(+f,0\right)$ and $\left(-f,0\right)$, the major axis vertices at $\left(+a,0\right)$ and $\left(-a,0\right)$, the minor axis vertices at $\left(0,+b\right)$ and $\left(0,-b\right)$. It also shows how the sum of the distances from any point on the ellipse to the two foci is a constant, and how the eccentricity is determined by relating one of the foci to a line $D$ called the directrix.

Source

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https://en.wikipedia.org/wiki/Ellipse Wikipedia Public domain.