Question

which undefined term is used to define an angle?
line
plane
point
ray

Explanation:

An angle is defined as the figure formed by two rays sharing a common endpoint. The undefined terms in geometry are point, line, and plane. A ray is a defined term (formed from a point and a line segment). To define an angle, we use the undefined term "point" as the common endpoint of the two rays (and also lines/rays involve points). Let's analyze each option:

• Line: A line is undefined, but an angle is about two rays (or lines) meeting at a point, so the key undefined term for the vertex of the angle is point, not line alone.
• Plane: A plane is an undefined term related to a flat surface, not directly to defining an angle.
• Point: The common endpoint of the two rays (that form the angle) is a point, and point is an undefined term. This is crucial as the angle's vertex is a point.
• Ray: Ray is a defined term (since it's defined using a point and a line segment), so it's not an undefined term.

Answer:

point