Eccentricity of Ellipse is Interfocal Distance over Major Axis

Theorem

Let $K$ be a ellipse such that:

$a$ denotes the length of semi-major axis of $K$

$c$ denotes the distance between the foci of $K$

$e$ denotes the eccentricity of $K$.

Then:

$e = \dfrac c {2 a}$

Proof

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Sources

• 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): ellipse
• 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): ellipse