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acute angle
an angle whose measure is between 0° and 90°

acute triangle
a triangle whose three interior angles are all acute

adjacent angles
two angles that have a common vertex and a common side between them

altitude of cone (pyramid)
the line segment from the vertex of the cone perpendicular to the plane of the base

altitude of cylinder (prism)
a line segment between and perpendicular to each of the two bases

altitude of parallelogram
a line segment drawn perpendicularly from a vertex to a nonadjacent side (known as the related base) of the parallelogram

altitude of trapezoid
a line segment drawn perpendicularlyfrom a vertex to the remaining parallel side

altitude of triangle
a line segment drawn perpendicularly from a vertex of the triangle to the opposite side of the triangle; the length of the altitude is the height of the triangle

angle
the plane figure formed by two rays that share acommon endpoint

base of isosceles triangle
the side of the triangle whose length is unique; the side opposite the vertex

bases of trapezoid
the two parallel sides of the trapezoid

bisector of angle
a ray that separates the given angle into two smaller, congruent angles

center of circle
the interior point of the circle whose distance from all points on the circle is the same

center of regular polygon
the common center of the inscribed and circumscribed circles of the regular polygon

center of sphere
the interior point of the sphere whose distance from all points on the sphere is the same

central angle of circle
an angle whose vertex is at the center of the circle and whose sides are radii of the circle

central angle of regular polygon
an angle whose vertex is at the center of the regular polygon and whose sides are two consecutive radii of the polygon

centroid of triangle
the point of concurrence for the three medians of the triangle

chord of circle
any line segment that joins two points on the circle

angle of depression (elevation)
acute angle formed by a horizontal ray and a ray determined by a downward (an upward) rotation

apothem of regular polygon
any line segment drawn from the center of the regular polygon perpendicular to one of its sides

arc
the segment (part) of a circle determined by two points on the circle and all points between them

area
the measurement, in square units, of the amount of region within an enclosed plane figure

auxiliary line
a line (or part of a line) added to a drawing tohelp complete a proof or solve a problem

base
a side (of a plane figure) or face (of a solid figure) to which an altitude is drawn

base angles of isosceles triangle
the two congruent angles of the isosceles triangle

circle
the set of points in a plane that are at a fixed distance from a given point (the center of the circle) in the plane

circumcenter of triangle
the center of the circumscribed circle of a triangle; the point of concurrence for the perpendicular bisectors of the three sides of the triangle

circumference
the linear measure of the distance around a circle

circumscribed circle
a circle that contains all vertices of a polygon whose sides are chords of the circle

circumscribed polygon
a polygon whose sides are all tangent to a circle in the interior of the polygon

collinear points
points that lie on the same line

common tangent
a line (or segment) that is tangent to more than one circle; can be a common external tangent or a common internal tangent

complementary angles
two angles whose sum of measures is 90°

concave polygon
a polygon in which at least one diagonal lies in the exterior of the polygon

concentric circles (spheres)
two or more circles (spheres) that have the same center

conclusion
the “then” clause of an “If, then” statement; the part of a theorem indicating the claim to be proved

concurrent lines
three or more lines that contain the same point

congruent
refers to figures (such as angles) that can be made to coincide

converse
relative to the statement “If P, then Q,” this statement has the form “If Q, then P”

convex polygon
a polygon in which all diagonals lie in the interior of the polygon

coplanar points
points that lie in the same plane

corollary
a theorem that follows from another theorem as a“byproduct”; a theorem that is easily proved as the consequence of another theorem

cosecant
in a right triangle, the ratio

cosine
in a right triangle, the ratio

cotangent
in a right triangle, the ratio

cube
a right square prism whose edges are congruent

cyclic polygon
a polygon that can be inscribed in a circle

cylinder (circular)
the solid generated by all line segments parallel to the axis of the cylinder and which contain corresponding end points on the two congruent circular bases

decagon
a polygon with exactly 10 sides

deduction
a form of reasoning in which specific conclusions are reached through the use of established principles

degree
the unit of measure that corresponds to of a complete revolution; used with angles and arcs

diagonal of polygon
a line segment that joins two nonconsecutive vertices of a polygon

diameter
any line segment that joins two points on a circle (or sphere) and that also contains the center of the circle (or sphere)

dodecagon
a polygon that has exactly 12 sides

dodecahedron (regular)
a polyhedron that has exactly12 faces that are congruent regular pentagons

edge of polyhedron
any line segment that joins two consecutive vertices of the polyhedron (includes prisms and pyramids)

equiangular polygon
a type of polygon whose angles are congruent (equal)

equilateral polygon
a type of polygon whose sides are congruent(equal)

equivalent equations
equations for which the solutions are the same

extended proportion
a proportion that has three or moremembers, such as

extended ratio
a ratio that compares three or more numbers,such as a:b:c

exterior
refers to all points that lie outside an enclosed(bounded) plane or solid figure

exterior angle of polygon
an angle formed by one side of the polygon and an extension of a second side that has a common endpoint with the first side

extremes of a proportion
 the first and last terms of a proportion; in ,
 a and d are the extremes

face of polyhedron
any one of the polygons that lies in a plane determined by the vertices of the polyhedron; includes base(s) and lateral faces of prisms and pyramids

geometric mean
the repeated second and third terms of certain proportions; in ,
b is the geometric mean of a and c

height
the length of the altitude of a geometric figure

heptagon
a polygon that has exactly seven sides

hexagon
a polygon that has exactly six sides

hexahedron (regular)
a polyhedron that has six congruent square faces; also called a cube

hypotenuse of right triangle
the side of a right triangle that lies opposite the right angle

hypothesis
the “if” clause of an “If, then” statement; the part of a theorem providing the given information

icosahedron (regular)
a polyhedron with 20 congruent faces that are equilateral triangles

incenter of triangle
the center of the inscribed circle of a triangle; the point of concurrence for the three angle bisectors of the angles of the triangle

induction
a form of reasoning in which a number of specific observations are used to draw a general conclusion

inscribed angle of circle
an angle whose vertex is on a circle and whose sides are chords of the circle

inscribed circle
a circle that lies inside a polygon in such away that the sides of the polygon are tangents of the circle

inscribed polygon
a polygon whose vertices all lie on a circle in such a way that the sides of the polygon are chords of the circle

intercepted arc
the arc (an arc) of a circle that is cut off in the interior of an angle (or related angle)

intercepts
the points at which the graph of an equation intersects the axes

interior
refers to all points that lie inside an enclosed (bounded) plane or solid figure

interior angle of polygon
any angle formed by two consecutive sides of the polygon in such a way that the angle lies in the interior of the polygon

