# Antiparallel (mathematics)

The lead section of this article may need to be rewritten. (July 2014) (Learn how and when to remove this template message) |

In geometry, **anti-parallel lines** can be defined with respect to either lines or angles.

## Contents

## DefinitionsEdit

Given two lines and , lines and are anti-parallel with respect to and if in Fig.1. If and are anti-parallel with respect to and , then and are also anti-parallel with respect to and .

In any quadrilateral inscribed in a circle, any two opposite sides are anti-parallel with respect to the other two sides (Fig.2).

Two lines and are antiparallel with respect to the sides of an angle if and only if they make the same angle in the opposite senses with the bisector of that angle (Fig.3).

## Antiparallel vectorsEdit

In a Euclidean space, two directed line segments, often called *vectors* in applied mathematics, are **antiparallel**, if they are supported by parallel lines and have opposite directions.^{[1]} In that case, one of the associated Euclidean vectors is the product of the other by a negative number.

## RelationsEdit

- The line joining the feet to two altitudes of a triangle is antiparallel to the third side.(any cevians which 'see' the third side with the same angle create antiparallel lines)
- The tangent to a triangle's circumcircle at a vertex is antiparallel to the opposite side.
- The radius of the circumcircle at a vertex is perpendicular to all lines antiparallel to the opposite sides.

## ReferencesEdit

**^**Harris, John; Harris, John W.; Stöcker, Horst (1998).*Handbook of mathematics and computational science*. Birkhäuser. p. 332. ISBN 0-387-94746-9., Chapter 6, p. 332

## SourcesEdit

- A.B. Ivanov, Encyclopaedia of Mathematics - ISBN 1-4020-0609-8
- Weisstein, Eric W. "Antiparallel." From MathWorld—A Wolfram Web Resource. [1]