Geometry. a fixed line used in the description of a curve or surface.
Archaic. a directress.
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How to use directrix in a sentence
The surface formed by revolving the catenary about its directrix is named the alysseide.
Since the tension is measured by the height above the directrix these two catenaries have the same directrix.
The only surface of revolution having this property is the catenoid formed by the revolution of a catenary about its directrix.
Hence T, the point of intersection of Pp and Qq, must be the centre of similitude and must be on the common directrix.
If the directrix, focus and eccentricity be given, we may employ the general method for constructing a conic.
British Dictionary definitions for directrix
geometry a fixed reference line, situated on the convex side of a conic section, that is used when defining or calculating its eccentricity
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Scientific definitions for directrix
A straight line used in generating a curve such as a parabola.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.