One of the mysteries of the English language finally explained.
A fixed line used in describing a curve or surface.
- ‘In the following, the feet of perpendiculars dropped from points A, B, etc. on the parabola to the directrix will be denoted A ’, B ’, etc.’
- ‘It is the pedal curve of the parabola where the pedal-point is taken as the reflection of the focus in the directrix.’
- ‘The directrix is used in the construction, which follows, mutatis mutandis, the same course as the above construction in the case of the ellipse.’
- ‘The hyperboloid is then indented and connected to a series of hyperbolic paraboloids through their common directrices, thereby leading the ensemble toward the four load-bearing points to the ground.’
- ‘The focus and directrix of a hyperbola were considered by Pappus.’
Early 18th century: from medieval Latin, literally ‘directress’, based on Latin dirigere ‘to guide’.
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