Basic Concepts in Geometry

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Point, Line, and Plane are the three basic undefined terms in geometry. These are fundamental concepts that we do not define but use to build all other geometric concepts. Lines and planes are sets of

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The distance between points D and E is 2 units. Distance = larger coordinate - smaller coordinate = 3 - 1 = 2. This is written as d(D,E) = 2 or l(DE) = 2.

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To find distance between two points: 1) Identify their coordinates, 2) Subtract the smaller coordinate from the larger coordinate. The result is always positive. For example: if A is at -5 and B is at

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If P, Q, R are three distinct collinear points, then Q is between P and R if d(P,Q) + d(Q,R) = d(P,R). This is written as P-Q-R. The point Q divides the segment PR into two parts.

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A line segment AB (seg AB) is the union of point A, point B, and all points between A and B. Points A and B are called the endpoints of the segment. The length of seg AB is the distance between its en

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Ray AB is the union of all points on seg AB and all points P such that A-B-P. Point A is the starting point. Line AB is the union of ray AB and its opposite ray. A ray has one starting point, while a

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Two segments are congruent if they have equal lengths. If l(AB) = l(CD), then seg AB ≅ seg CD. Congruent segments have the same measure but may be in different positions.

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1) Reflexivity: seg AB ≅ seg AB (every segment is congruent to itself), 2) Symmetry: If seg AB ≅ seg CD, then seg CD ≅ seg AB, 3) Transitivity: If seg AB ≅ seg CD and seg CD ≅ seg EF, then seg AB ≅ se