Examples:

quadrangle or quadrilateral  A polygon with four sides.

quotient  See division.

radius  A line segment from the center of a circle (or sphere) to any point on the circle (or sphere); also, the length of such a line segment.

random  Without any pattern or preference.

random number  A number that has the same chance of being chosen as any other number.

random sample  A sample taken from a population in a way that gives all members of the population the same chance of being selected.

range  The difference between the maximum and minimum values in a set of data.

rate  A comparison of two quantities with unlike units.  For example, a speed such as 55 miles per hour compares distance with time.

rate diagram  A tool used to solve rate problems.

rate table  A table displaying rate information.

rate unit  A compound unit used to describe a rate, made up of two different units.  For example, miles per hour, dollars per pound, and words per minute are all rate units.

ratio  A comparison of two quantities with like units.  Ratios can be expressed with fractions, decimals, percents, or words; or they can be written with a colon between the two numbers being compared.  For example, if a team wins 3 games out of 5 games played, the ration of wins to total games is 3/5, 0.6, 60%, 3 to 5, or 3:5 (read “three to five”).

rational number  Any number that can be represented in the form a/b, where a and b are integers and b is not 0.  Also, any number that can be represented by a terminating decimal or repeating decimal.  2/3, -2/3, 0.5, -0.5, and 0.333 … are rational numbers.

ray  A straight path that extends infinitely in one direction from a point, called its endpoint.

reaction time  The amount of time it takes to respond to a stimulus.

real number  Any rational or irrational number.

recall survey  A type of survey used to collect information on past behavior, habits, and activities.  Participants in such a survey are asked to recall information from memory.

reciprocal  See multiplicative inverses.

rectangle  A parallelogram with four right angles.

rectangular array  A rectangular arrangement of objects in rows and columns in which each row has the same number of parts and each column has the same number of parts.  See also square array.

rectangular land survey system  A land survey that uses reference lines parallel to the north-south meridians of longitude and the east-west parallels of latitude to divide the land.  It outlines pieces of land shaped like rectangles.

rectangular method  A method for finding area, in which rectangles are used to surround a figure or parts of a figure.  All the areas that are calculated are either areas of rectangles or of triangular halves of rectangular regions.

reference frame  A system of numbers, letters, or words to show quantities with reference to a zero point.  Examples of reference frames are number lines, time lines, calendars, thermometers, maps, and coordinate systems.

reflection  A transformation in which a figure “flips” so that its image is the reverse of the original (preimage).

reflex angle  An angle whose measure is between 180° and 360°.

regular polygon  A convex polygon in which all the sides are the same length and all the angles have the same measure.

regular polyhedron  A polyhedron with faces that are all congruent regular polygons and their interiors.  There are five regular polyhedrons:

tetrahedron          4 faces, each formed by an equilateral triangle
cube                    6 faces, each formed by a square
octahedron          8 faces, each formed by an equilateral triangle
dodecahedron     12 faces, each formed by a regular pentagon
icosahedron         20 faces, each formed by an equilateral triangle

regular tessellation  A tessellation made up of only one kind of regular polygon.

relation symbol  A symbol used to express the relationship between two quantities.  The symbols used in number sentences are: = for “is equal to”, ¹ for “is not equal to”, < for “is less than”, > for “is greater than”, £ for “is less than or equal to”, ³ for “is greater than or equal to”.

remainder  See division.

repeating decimal  A decimal which has one digit, or a group of digits, that is repeated without end.  For example, 0.3333… and 0.147147… are repeating decimals.  See decimal.

revolution  Movement in a circle or a curve around some point.

rhombus  A parallelogram whose sides are all the same length.  The angles are usually not right angles, but they may be right angles.

right angle  An angle with a measure of 90 degrees.

right triangle  A triangle that has a right angle.

rotation  A transformation in which a figure “turns” around a center point or axis.

rotation symmetry  Property of a figure that can be rotated around a point in such a way that the resulting figure (the image) exactly matches the original figure (the preimage).  The rotation must be more than 0 degrees, but less than 360 degrees.  If a figure has rotation symmetry, its order of rotation symmetry is the number of different ways it can be rotated to match itself exactly.  “No rotation” is counted as one of the ways.

rounding  Replacing a number with a nearby number that is easier to work with or better reflects the precision of the data.  12,964 rounded to the nearest thousand is 13,000.

row  See cell.

rubber-sheet geometry  See topology.