To determine why a ray is considered a defined term, let's explore the characteristics of a ray and evaluate the given options one by one.
1. Has no width or depth: While it is true that a ray has no width or depth, this characteristic alone does not explain why it is a defined term. This is more about the properties of a ray rather than the reason it is defined.
2. Is described using an undefined term: A ray is described using points, specifically an initial point and another point that it passes through. Since points are considered undefined terms in geometry, describing a ray using points is consistent with how defined terms are developed from undefined terms.
3. Has an informal definition: This option implies that the definition of a ray is not precise or rigorous, which is not accurate. In geometry, a ray has a formal definition involving an endpoint and all points extending infinitely in one direction from that endpoint.
4. Travels forever in one direction: While true, this is a description of the behavior of a ray rather than an explanation for why it is considered a defined term. This characteristic is more about how a ray extends infinitely in one direction from its initial point.
Based on this evaluation:
The reason a ray is a defined term is primarily due to it being described using an undefined term. In geometry, points are undefined terms, and a ray is defined using a point as its starting point and another point to determine its direction.
Therefore, the correct answer is:
Is described using an undefined term.
