A ray or half-line is a part of a line consisting of all points of this line lying on one side of a fixed point of this line, and this point itself, called the beginning of the ray or the starting point of a half-line.
A ray is indicated, as well as a straight line, by lowercase Latin letters a, b, c, ..., or by a record of the form [AB), where A is the beginning of the ray and B is the point lying on the ray.
Different rays of the same straight line, having a common starting point, are called additional half-lines. Semi-direct AC and AB are called optional.
Axiom At any half-line from its starting point, you can postpone a segment of a given length, and only one. |
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Planometry
Terms: Planometry