Angle ∠BAC might refer to any of four angles: the clockwise angle from B to C, the anticlockwise angle from B to C, the clockwise angle from C to B, or the anticlockwise angle from C to B, where the direction in which the angle is measured determines its sign (see Positive and negative angles). However, in many geometrical situations it is obvious from context that the positive angle less than or equal to 180 degrees is meant, and no ambiguity arises. Otherwise, a convention may be adopted so that ∠BAC always refers to the anticlockwise (positive) angle from B to
Antiparallel (mathematics) and only if they make the same angle formula_18 in the opposite senses with the bisector of that angle (Fig.3). 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. In that case, one of the associated Euclidean vectors is the product of the other by a negative number. Antiparallel (mathematics) In geometry, anti-parallel lines can be defined with respect to either lines or angles. Given two lines formula_1 and formula_2, lines formula_3 and formula_4 are anti-parallel with respect to formula_1 and formula_2