Distance of Directrix of Ellipse from Center

Theorem

Let $K$ be a ellipse with semi-major axis $a$ and eccentricity $e$.

The distance of the directrix of $K$ from the center of $K$ is equal to $\dfrac a e$.

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