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decagon A ten-sided polygon.

decimal notation  The notation in which numbers are written using ten digits and each place stands for a power of ten.

Ex: 34 means 3 tens and 4 ones.

decimal system  The system in which numbers are written in decimal notation.

deduction The process of making justified conclusions.

definition A description that clearly and uniquely specifies an object or a class of objects.

degree A unit of measurements equal to 1/360 of a complete circle.

degree measure of a major arc ACB of circle O
360 - the measure of arc AB. degree measure of minor arc AB of circle O The measure of the central angle AOB.

denominator  The divisor, or bottom number, in a fraction.
b in the fraction a/b.

Remember that a fraction is a division problem. The top number, the numerator, is the number being divided, the fraction bar is the division symbol, and the bottom number, the denominator, is the number of equal parts that you are dividing the top into.

EX: Say you have \$24/8. Twenty four dollars divided into 8 equal parts means there will be \$3 in each part.

EX: If the fraction has a numerator smaller than its denominator you might get a problem like this: \$3/4. Three dollars divided into 4 equal parts. Each of these parts will have less than one dollar in it, in this case \$.75.

*If the numerator is greater than the denominator, the fraction is > 1.
**If the numerator is less than the denominator, the fraction is < 1.

dense The property that between two elements of the set there is at least one other element of the set. The set of Real Numbers is dense because between any two numbers you can always find another number, just enlarge the view.

diagonal of a polygon  A segment that connects two vertices of the polygon but is not a side of a polygon. NOTE: A triangle has no diagonals.

diameter of a circle or sphere A segment connecting two points on the circle or sphere and containing the center of the circle or sphere, also the length of that segment. difference The answer to a subtraction problem.

digit One of the ten symbols, 0 1 2 3 4 5 6 7 8 9, used to write numbers from zero to nine.

dihedral angle The angle formed by the union of two half-planes with the same edge. dimensions  The lengths of the sides of a rectangle. The number of rows and the number of columns in a rectangular array.

dimensions of a box The lengths of three edges of the box which meet at the same vertex. Usually referred to as length, width, and height.

dimensions of a rectangle The lengths of two sides of the rectangle which meet at a single vertex. Usually referred to as length and width.

direction of translation The direction given by the vector from any preimage point to its image point in a translation. directly congruent Figures which are congruent and have the same orientation.

discrete figure A figure made up of points with space between them.

discrete geometry The study of points as dots separate from each other, and lines made up of these points.

display The area on a calculator where numbers appear.

distance between two parallel lines The length of a segment that is perpendicular to the lines with an endpoint on each of the lines. distance between two parallel planes The length of a segment perpendicular to the planes with an endpoint in each plane.

distance between two points The absolute value of the difference of their coordinates on a coordinatized line. distance from a point to a line The length of the perpendicular segment connecting the point to the line. distance from a point to a plane The length of the perpendicular segment from the point to the plane. Distributive Property of Multiplication over Addition For any numbers a, b, and x : ax + bx = ( a + b )x and x( a + b) = xa + xb. Also called distributivity. Distributive Property of Multiplication over Subtraction   For any numbers a, b, and x : ax + bx = ( a - b )x and ( a - b ) = ax - bx. Also called distributivity. dividend  The number in a quotient which is being divided; a is the dividend in a/b. divisor The number by which you divide in a quotient; b is the divisor in a/b.    Also a number that exactly divides into another number, referred to as a factor. dodecagon A polygon with twelve sides.

dodecahedron A polyhedron with twelve faces.

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