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Welcome to Geometry
Unit 1 Vocabulary
Undefined Terms
• Point
• In Euclidean geometry, a point is undefined.
You can think of a point as a location. A point
has no size. You can represent a point by a dot
and name it by a capital letter, such as A.
• Example
• P
Undefined Terms
• Line
• In Euclidean geometry, a line is undefined. You can
think of a line as a straight path that extends in two
opposite directions without end and has no thickness.
A line contains infinitely many points. In spherical
geometry, you can think of a line as a great circle or a
sphere. You can name a line by any two points on the
line, such as “line AB” or “line BA,” or by a single lower
case letter, such as “line L.” A line has a measure of
180 degrees.
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Undefined Terms
• Plane
• In Euclidean geometry, a plane is undefined. You can think of a
plane as a flat surface that extends without end and has no
thickness. A plane contains infinitely many lines. You can name a
plane by at least three points in the plane that do not all lie on the
same line, such as “plane ABC,” or a capital letter such as “plane P.”
• Example
Collinear points
Collinear points lie on the same line.
Coplanar
Coplanar figures are figures in the same plane.
Space
Space is the set of all points.
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One-dimension
Two-dimension
Three-dimension
N-dimension
• Line Segment
A segment is the part of a line that consists of two
points, called endpoints, and all points between
them. You can name a segment by its two endpoints,
such as “segment DE” or “segment ED.”
Ray
A ray is the part of a line that consists of one endpoint
and all the points of the line on one side of the
endpoint. You can name a ray by its endpoint and
another point on the ray, such as “ray AB”. The order of
points indicates the ray’s direction. You cannot name
this as “ray BA.”
Opposite rays
Opposite rays are collinear rays with the
same endpoint. They form a line.
Postulate
A postulate, or axiom, is an
accepted statement of fact.
Intersection
The intersection of two or more
geometric figures is the set of points
the figures have in common.
Coordinate
The coordinate of a point is its distance and direction
from the origin of a number line. The coordinates of a
point on a coordinate plane are in the form (x,y),
where x is the x-coordinate and y is the y-coordinate.
Congruent segments
Congruent segments are segments that have the
same length.
Midpoint of a segment
A midpoint of a segment is the point
that divides the segment into two
congruent segments.
Segment bisector
A segment bisector is a line, segment,
ray, or plane that intersects a segment
at its midpoint.
Angle
An angle is formed by two rays with the
same endpoint. The rays are the sides of
the angle and the common endpoint is the
vertex of the angle
Vertex
The endpoint of the angle is the vertex
of the angle.
Acute angle
An acute angle is an angle whose
measure is between 0 and 90.
Right angle
A right angle is an angle whose
measure is 90.
Obtuse angle
An obtuse angle is an angle whose
measure is between 90 and 180.
Straight angle
A straight angle is an angle whose
measure is 180.
Congruent angles
Congruent angles are angles that
have the same measure.
Adjacent angle
Adjacent angles are two coplanar angles
that have a common side and a common
vertex but no common interior points.
Vertical angles
Vertical angles are two angles
whose sides form two pairs of
opposite rays.
Complementary angles
Two angles are complementary
angles if the sum of their measures
is 90.
Supplementary angles
Two angles are supplementary if the sum
of their measures is 180.
Linear pair
A linear pair is a pair of adjacent angles
whose non-common sides are opposite
rays. Angles that are a linear pair are
supplementary angles.
Angle bisector
An angle bisector is a ray that
divides an angle into two
congruent angles.