Geometry: Unit 1 Vocabulary TERM DEFINITION GEOMETRIC FIGURE. Cannot be defined by using other figures.

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1 Geometry: Unit 1 Vocabulary 1.1 Undefined terms Cannot be defined by using other figures. Point A specific location. It has no dimension and is represented by a dot. Line Plane A connected straight path. It has no thickness and it continues forever in both directions. A flat surface. It has no thickness and it extends forever in all directions. Line Segment (segment) A portion of a line consisting of two points, endpoints, and all points between them. Ray Coplanar A portion of a line that starts at a point, the endpoint, and continues forever in one direction. Points that lie in the same plane. Collinear Points that lie on the same line. Parallel Lines that lie in the same plane but do not intersect.

2 Midpoint A point that divides a line segment into two segments that have the same length. Segment Bisector A line, ray, or other figure that passes through the midpoint of a segment. 1.2 Angle A figure formed by two rays with the same endpoint. Vertex The common endpoint of the angle. Sides The rays of the angles. Degrees ( ) Acute Angle Common measurement unit for measuring distance around a circular arc. An angle measure that is between 0 and 90 degrees. Right Angle An angle that measures 90 degrees. Obtuse Angle An angle measure that is between 90 and 180 degrees. Straight Angle An angle that measures 180 degrees.

3 Angle Bisector A ray that divides an angle into two angles that both have the same measure. 1.3 Transformation A function that changes the position, shape, and/or size of a figure. Pre-image A figure that is used as the input of a transformation. Image A figure that is the output of a transformation. Rigid Motion or Isometric A transformation that changes the position of a figure without changing the size or shape of the figure. 1.4 Conjecture A statement that is believed to be true. Inductive or deductive reasoning can be used to prove a conjecture. Inductive Reasoning Deductive Reasoning The process of reasoning that a rule or statement is true because specific cases are true. The process of using logic to draw conclusions. Postulate A statement that is accepted as true without proof. Theorem A statement that you can prove is true using a series of logical steps.

4 Counterexample An example that shows a conjecture to be false. Conditional Statement Complementary Angles A statement that can be written in the form If p, then q. Two angles whose measures add up to 90. Supplementary Angles Angles whose measures add up to 180. Linear Pair A pair of adjacent angles whose non-common sides are opposite rays. 2.1 Vector A quantity that has both direction and magnitude. Initial Point The starting point of a vector. Terminal Point The ending point of a vector. Translation A transformation along a vector such that the segment joining a point and its image has the same length as the vector and is parallel to the vector.

5 2.2 Perpendicular Lines Lines that intersect at right angles. Perpendicular Bisector A line perpendicular to a line segment at the segment s midpoint. Reflection Maps a point P to its image P, across the line of reflection l. If P is not on line l, then line l is the perpendicular bisector of PP. If P is on line l, then P = P. 2.3 Rotation A transformation around point P, the center of rotation. Every point and its image are the same distance from P. All angles with vertex P formed by a point and its image have the same measure, which is the angle of rotation. 2.4 Symmetry If a rigid motion exits that maps the figure onto itself, then there is symmetry. Line of Symmetry or Reflectional Symmetry Rotational Symmetry A reflection maps the figure onto itself folding over the line of symmetry. A rotation maps the figure onto itself. The angle of rotational symmetry is greater than 0 but less than or equal to 180. It is the smallest angle that maps onto itself.

6 3.3 Corresponding Parts of Congruent Figures are Congruent If two figures are congruent, then corresponding sides are congruent and corresponding angles are congruent.