Definition:Directrix of Ruled Surface

Definition

A directrix is a curve which defines the generators of a ruled surface.

Directrix of Cone

Let $K$ be a cone.

Let $B$ be the base of $K$.

The boundary of $B$ is known as the directrix of $K$.

Directrix of Cylindrical Surface

The directrix of a cylindrical surface $S$ is the curve $C$ through which pass all the parallel straight lines forming $S$.

Also see

• Results about directrices of ruled surfaces can be found here.

Linguistic Note

The plural of directrix is directrices.

Sources

• 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): directrix: 2. (plural directrices)
• 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): directrix: 2. (plural directrices)