directrix

/dih-rek-triks, dahy-/US // dɪˈrɛk trɪks, daɪ- //UK // (dɪˈrɛktrɪks, daɪ-) //

指示器,指示牌,指示者,指示员

Definitions

n.名词 noun

1
plural di·rec·trix·es, di·rec·tri·ces [dih-rek-tri-seez, dahy-, dahy-rek-trahy-seez]. /dɪˈrɛk trɪˌsiz, daɪ-, ˌdaɪ rɛkˈtraɪ siz/.
- : Geometry. a fixed line used in the description of a curve or surface.
- : Archaic. a directress.

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.