Geometry for Elementary School/Glossary

What next? Conventions Below is a glossary of words used in this book. A Acute angle An angle that is less than 90 degrees Adjacent angle When two angles share the same vertice, and one same but opposite arm Adjacent angles on a straight line When angles add up to 180 degrees and are adjacent Altitude The line drew from the base to the tip of the triangle Angle sum of triangles The angle sum of triangles is 180 degrees Angles at a point When angles add up to 360 degrees and are adjacent Arc A part of a circumference Area How big a plane shape is Arms One of the two rays in an angle Axiom Similar to a theorem, except it is thought up rather than proved B Base Any side of the triangle C Centre The middle of a shape, especially a circle Chord A line that goes from one point of the circumference to another with touching the centre Circle A shape with infinite loci that are the same distance from the circle Collinear When two points are on the same line Complementary angles Angles that add up to 90 degrees Cone A shape with a circle as a base and a pointy top Congruence The property of two different figures being of the same size and shape Coplanar When two points or lines are on the same plane Cross-section The shape formed after cutting a solid Cube A hexahedron with all edges equal Cuboid A rectangular prism Curved surface A surface that is not smooth Cylinder A shape with parallel, congruent circles as bases D Decagon A polygon with ten sides Decahedron A polyhedron with ten sides Degree The unit used for measuring angles Diameter The line that goes from one point of the circumference, through the centre and to another side of the circumference Dimension The kind of space that a shape is in. If a shape has 2-dimensions, it has width and length; if it has three, it has width, height and length. Dissecting A method of finding the area of a polygon Dodecagon A polygon with twelve sides Dodecahedron A polyhedron with twelve sides E Edge The edge of a solid Ellipse A shape that looks like an oval Equiangular When a polygon's angles are identical Equilateral When a polygon's sides are identical Euler's formula Faces + Vertices - Edges = 2 F Face A plane figure on a solid Filling A method of finding the area of a polygon Finite Not unlimited Focus One of the two points in the middle of a ellipse G H Height The third dimension of a solid or the altitude of a triangle Heptagon A polygon with seven sides Heptahedron A polyhedron with seven sides Hexagon A polygon with six sides Hexahedron A polyhedron with six sides I Intersect When two or more lines, planes, etc., cross each other Irregular Not regular Isosceles trapezium A trapezium with two sides equal Isosceles triangle A triangle with two sides identical J K Kite A quadrilateral where two pairs of adjacent sides are equal and one pair of opposite angles is equal L Line A series of colinear points which extends in both directions forever; has no width Line of intersection The line formed when two planes intersect Line segment A line with a fixed length, with two endpoints M N Nonagon A polygon with nine sides Nonahedron A polyhedron with nine sides O Obtuse angle An angle between 90 and 180 degrees Octagon A polygon with eight sides Octahedron A polyhedron with eight sides P Parallel line Two lines that never intersect Parallelogarm A quadrilateral where opposite sides and angles are equal Pentagon A polygon with five sides Pentahedron A polyhedron with five sides Perimeter The sum of a plane shape's sidse Perpendicular line When two lines intersect to create right angles Pi The ratio of the diameter and the circumference Plane A plane that extends in all directions forever; has no thickness Point A dot that is so small that its height and width are actually zero Point of intersection The point formed when two lines intersect Polygon A shape that has straight sides Postulate See Axiom Prism A shape with parallel, congruent bases Protractor A tool for measuring angles Pyramid A shape with a base and a pointy top Q Quadrilateral A polygon with four sides R Radius The line that goes from the centre to one point of the circumference Ray A line that only extends in one direction forever Rectangle A quadrilateral opposite sides are equal and all angles are 90 degrees Reflex angle An angle between 180 and 360 degrees Regular When a polygon is both equilateral and equiangular, or when a solid satisfies all the criteria for regular polyhedra Rhombus A quadrilateral where all sides are equal and opposite angles are equal Right angle An angle of 90 degrees Right-angled trapezium A trapezium with a right angle Round angle An angle of 360 degrees S Scalene trapezium A trapezium with neither a right angle nor equal sides Semicircle Half a circle Septagon See Heptagon Septahedron See Heptahedron Sphere A ball Splitting See Dissecting Square A regular quadrilateral where all sides are equal and all angles are equal Straight angle An angle of 180 degrees Supplementary angles Angles that add up to 180 degrees T Tetrahedron A polyhedron with four sides Total surface area The total area of all faces of a solid Trapezium A quadrilateral where there is one pair of parallel sides Trapezoid See Trapezium Triangle A polygon with three sides U Undecagon A polygon with eleven sides Undecahedron A polyhedron with eleven sides Uniform cross-sections The identical shapes formed after cutting the solid in the same direction; only on prisms and cylinders V Vertex A corner in things such as angles, plane figures and solids Vertically opposite angles When angles share the same vertex and their opposite arms are on the same straight line; are always the same size. Volume How big a solid is W X Y Z