Download Definitions of Key Geometric Terms

Definitions of Key Geometric
Terms
A quick review of material covered in
Math A
La Salle Academy, Mrs. Masullo
Point
• A position in space.
• A point has no dimensions.
• A point is represented by a dot and named
with a capital letter.
Line
• An infinite set of points extending in two
directions.
• A line is named using two of its points,
such as AB with a double arrow above it,
or by using a single lowercase script letter.
Plane
• A set of points extending infinitely in all
directions.
• A plane is named with a capital letter.
Collinear
• Points that are contained on the same line.
Coplanar
• Points that are contained on the same
plane.
Postulate
• A statement that is assumed to be true.
Theorem
• A statement that can be proven by
deductive reasoning.
Congruent
• If two figures are congruent, they have the same
size and shape.
• The symbol for congruence is
meaning that the figures are equal (=) in measure
and similar (~) in shape.

Angle
• The union of two rays that share a common
endpoint.
• The two rays form the sides of the angle and the
intersection of the two rays is the vertex of the
angle.
• An angle can be named by a capital letter
located at its vertex; by three capital letters, the
middle letter being the vertex; by a lowercase
letter or a number placed inside the angle.
Bisector
• A bisector of an angle is a ray whose
endpoint is the vertex of the angle, which
divides the angle into two congruent
angles.
• A bisector of a line segment is the line (or
part of a line) that intersects the segment
at its midpoint, dividing the segment into
two congruent line segments.
Adjacent Angles
• Two coplanar angles that share a common
vertex and a common side, but have no
interior points in common.
Complementary angles
• Two angles whose sum in degree
measures is 90.
Supplementary angles
• Two angles whose sum in degree
measures is 180.
• Complementary and supplementary
angles can be adjacent or nonadjacent.
A linear pair of angles
• A pair of two adjacent angles whose sum
is a straight angle.
Line Segment
• The set of points containing two points on
a line, called the endpoints of the line
segment, and all points on the line
between the endpoints.
• Because a line segment has two
endpoints, it has a definite length.
Midpoint
• The midpoint of a line segment is the point
that divides the segment into two
congruent segments.
Perpendicular lines
• Two lines that intersect to form right
angles.
Perpendicular bisector
• A line (or part of a line) that is
perpendicular to the line segment,
intersecting the line segment at its
midpoint.
Polygon
• A closed figure formed by coplanar line
segments that are joined at their
endpoints.
• A REGULAR polygon is both equilateral
and equiangular (all sides and all angles
are congruent).
Triangles classified by angles
• A polygon that has exactly three sides.
• An ACUTE triangle has three acute angles
(each measures < 90 deg.)
• A RIGHT triangle has one right angle and
two acute angles. The sides that form the
right angle are called legs, and the side
opposite the right angle is the hypotenuse.
• An OBTUSE triangle has one obtuse
angle (>90 deg.) and two acute angles.
Triangles classified by sides
• An EQUILATERAL triangle has three
congruent sides. An equilateral triangle is
also equiangular, so it is a regular polygon.
• An ISOSCELES triangle has at least two
congruent sides. The two congruent sides
are called legs, and the third sides is
called the base. Base angles are always
congruent.
• A SCALENE triangle has no congruent
sides.
Altitude
• The line segment drawn from any vertex of
an object , perpendicular to the opposite
side.
Median
• A line segment drawn from any vertex of
an object to the midpoint of the opposite
side.
Quadrilateral
• A four-sided polygon. The most important
quadrilaterals that we will study are
parallelograms and trapezoids.
Parallelogram
• A quadrilateral whose opposite sides are
parallel.
• A rectangle is a parallelogram containing
four right angles.
• A rhombus is a parallelogram with four
congruent sides.
• A square is a rhombus with four right
angles, or a rectangle with four congruent
sides.
Trapezoid
• A quadrilateral that has only one pair of
parallel sides, called bases. The nonparallel sides are the legs of the trapezoid.
• In an ISOSCELES trapezoid, the legs are
congruent.