# Definitions worksheet followup October 26 2016 Undefined terms

Definitions worksheet follow-up October 26, 2016

Undefined terms • Point – has no length or width or depth • Line – has length but no width or depth • Plane – has length and width but not depth • These are all ideal objects • They are also considered to be the basic object of geometry. Everything else can be defined using these three terms.

For example • Ray: part of a line that is bounded by a point on one side and unbounded on the other (note that online definitions are not always accurate) • Angle: two rays with a common endpoint

Naming conventions • Lines: • For lines always use two letters and an arrow above. The order of the letters does not matter. • Rays: • You also use two letters and a directed arrow above. The order of the letters does matter – you have to start with the endpoint.

• Line segments: • For line segments always use two letters and a line segment above. The order of the letters does not matter – line segments have no orientation • Angles: � ABC • The middle letter, B, has to be the vertex of the angle, where the rays BA and BC meet.

Skew lines • Where in the classroom can we see two skew lines?

Interior angle vs interior of an angle • An interior angle is an angle inside of a polygon • The interior of an angle is the area between the two rays that determine the angle • To further complicate things, there also alternate interior angles, but we won’t discuss those.

What is a right angle? • Define a right angle. • Define 90 degrees. • Note that you define one in terms of the other. But that is not possible. How else can we define what a right angle is?

Polygon definition • A closed plane figure made up of line segments joined end to end without crossing over. • The line segments are called the sides, and the endpoints are called the vertices of the polygon.

Other useful vocabulary • Adjacent • Diagonal

- Slides: 10