Definition:Parabola/Focus
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This page is about Focus of Parabola. For other uses, see focus.
Definition
Let $K$ be a parabola specified in terms of:
- a given straight line $D$
- a given point $F$
where $K$ is the locus of points $P$ such that the distance $p$ from $P$ to $D$ equals the distance $q$ from $P$ to $F$:
- $p = q$
The point $F$ is known as the focus of the parabola.
Linguistic Note
The word focus is of Latin origin, hence its irregular plural form foci.
It was introduced into geometry by Johannes Kepler when he established his First Law of Planetary Motion.
The word in Latin means fireplace or hearth, which is appropriate, considering the position of the sun.
The pronunciation of foci has a hard c, and is rendered approximately as foke-eye.
Beware the solecism of pronouncing it fo-sigh, which is incorrect.
Sources
- 2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.) ... (previous) ... (next): parabola