Basics of Geometry - PowerPoint PPT Presentation

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CK-12 Geometry FlexBooks describes Geometry basic concepts including points, angles, lines, and line segments.

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Basics of Geometry

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Study Guides

Big Picture

Geometry is founded upon some very important basic concepts. These include points, angles, lines, and line segments.

There are many postulates that govern the way that we can use these basic concepts. After the basic foundation is

created, we can progress to harder, more advanced geometry.

Key Terms

Undefined Term: A term that cannot be mathematically defined using other known words. The undefined terms point,

line, and plane are the building blocks of geometry.

Point: Location that has no size (no dimension).

Line: Infinite series of points in a row (1-dimensional). It has direction and location and is always straight.

Plane: Any flat, 2-dimensional surface.

Undefined terms can be used to define other geometric terms:

Segment: Portion of a line that is ended by two points.

Ray: Portion of line that has only one endpoint and extends infinitely in the other direction.

Endpoint: Point at the end of a segment or at the start of a ray.

Space: Set of all points expanding in three dimensions. It has no shape and no limits.

Collinear: Points that lie on the same line.

Coplanar: Points or lines that lie within the same plane.

Intersection: Point or set of points where lines, planes, segments, or rays cross (intersect).

Postulate: Basic rule that we can assume to be true. Also known as an axiom.

Drawing and Labeling




A capital label (A, L, F)


A lowercase letter (line g) or two points on the


line (PQ or QP)

A script capital letter (plane M) or three points


not on a line (plane ABC)

Think of a plane as a huge sheet of paper!

The two endpoints (AB or BA)


Endpoint and any other point on the ray (CD)






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Geometric Figures

Everything in the figure below occupies 3-dimensional

Point G is not on line h.


Points A, G, and K and lines h and i are coplanar.

Point E is not in plane j, but points E, A, and B are

coplanar even though the plane is not drawn in.

When drawing or labeling geometric figures, be very


Include arrows to show that the line extends to infinity.

Label points.

When labeling rays, make sure that the end point is

under the side without an arrow. KF is NOT the same

as FK.

Remember that arrows extend forever. In the figure below,

even though the intersection is not shown, the two lines

intersect at a point to the right.

The points A, B, and C are collinear.


Point C lies on line h between A and B. A point

is between two other points when they are in a


straight line; G is not between A and B.

•  CA

and CB are opposite rays that have a common

endpoint and form a line.

Basic Postulates

Postulates for Points, Lines, and Planes

Postulates for Intersection

1. There is exactly one line through any two points.

1. The intersection of any two distinct lines will be a

single point.

2. There is exactly one plane that contains any three

non-collinear points.

2. The intersection of two distinct planes is a line.

3. A line connecting points in a plane also lies within

the plane.

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