# 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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