# Math Dictionary d

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D

Data :A collection of facts, such as values or measurements.

Decade :A period of 10 years.

Decahedron :A solid shape with ten flat faces.

Decimal : The base used for it is 10.

Decagon : A polygon with ten sides.

Deductive proof : A formal proof based on logical argument that is justified using axioms and/or theorems.

Deductive reasoning : A process of showing that certain statements follow logically from agreed upon assumptions and proven facts; reasoning from the general to the specific.

Diagonal : A line segment that connects two non-consecutive vertices of a polygon.

Diameter : A chord of the circle that passes through the center of the circle.

Diameter of a sphere : A line segment that connects two points on the surface of a sphere and that passes through the center of the sphere.

Dihedral angle: An angle formed by two interesting planes.

Dilation : A transformation of the plane such that if O is a fixed point, k is a non-zero real number, and P’ is the image of point P, then O, P and P’ are collinear and OP'/OP = k.

Direct transformation : Any transformation of the plane that preserves orientation.Examples: A translation, rotation or a dilation.

Distance between two points: The length of the line segment joining the two points; a unique non-negative real number.

Distance between a point and a line : The length of the perpendicular segment from the point to the line.

Distance between two parallel lines : The length of a line segment drawn from any point on one line perpendicular to the second line.

Dodecahedron : A polyhedron that has twelve faces. A regular dodecahedron is one of the five. Platonic solids and has twelve regular pentagons as faces.

Dynamic geometry software : Computer or calculator software used to construct and manipulate geometric figures.

Math dictionary d

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