Glossary Entries

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Radii Radii is the plural form of radius.
Radius The radius of a circle is the distance from the center point to the edge of the circle. It is one half the length of the diameter.
Random Sample A sample obtained by a selection from a population, in which element of the population has an equal chance of being selected.
Random Variable A function on a probability space.
Range of a Data In statistics, it is the difference between the largest and the smallest values in a set of numbers. Example: Given the data: 2, 7, 3, 14, –1, 6, 34, –3 The range is 34 – (–3) = 37. In mathematics, it is the image of a function.
Range of a Function The set of output values of a function.
Rate Rates are ratios that compare quantities of different units. Examples: Miles per hour, price per pound, students per class, heartbeats per minute.
Rate of Change The amount the function's output increases or decreases for each unit of change in the input.
Rate of Interest The percent charged or paid for the use of money. Example:$1,000 was borrowed at an interest rate of 8.5% per year for a period of 18 months.The rate of interest is 8.5%.
Ratio A comparison of two numbers or two like quantities by division Example: The ratio of girls to boys is two to three, 2/3. 2 to 3, or 2:3.
Rational Number A number that can be expressed as a ratio or quotient of two non-zero integers. Rational numbers can be expressed as common fractions or decimals, such as 3/5 or 0.6. Finite decimals, repeating decimals, mixed numbers and whole numbers are all rational numbers. Nonrepeating decimals cannot be expressed in this way, and are said to be irrational.
Rationale An explanation using logical reasoning, mathematical principles, or arguments.
Raw Data Collected information before the application of statistical procedures.
Ray A part of a line that has one endpoint and goes on forever (infinitely) in one direction.
Real Number All of the numbers that are rational and irrational. All of these numbers can be expressed as decimals.
Real-World Problem Quantitative and spatial problems that arise from a wide variety of human experiences, applications to careers. These do not have to be highly complex ones and can include such things as making change, figuring sale prices, or comparing payment plans.
Reasonable Estimate An approximation of the result of a given problem or calculation using rational, logical procedures (e.g., rounding).
Reasonableness The justification that a particular solution to a problem is within logical estimates.
Reasoning Engaging in a process that leads to a conclusion or inference using known facts or assumptions.
Reciprocal The number that is used to multiply a given number to obtain an answer of 1 (e.g., 2/3x3/2=1, therefore 3/2 is the reciprocal of 2/3). (See multiplicative Inverse)
Rectangle A quadrilateral with four right angles.
Rectangular Coordinate System This system uses two (for a plane) or three (for space) mutually perpendicular lines (called coordinate axes) and their point of intersection (called the origin) as the frame of reference. Specific locations are described by ordered pairs or triples (called coordinates) that indicate distance from the origin along lines that are parallel to the coordinate axes.
Rectangular Prism A prism with six rectangular faces.
Reflection A transformation creating a mirror image of a figure on the opposite side of a line. A reflection is also called a flip. The line is the perpendicular bisector of each line segment connecting a point and its image.
Reflex Angle An angle that has a value greater than 180 degrees but less that 360 degrees.
Region A part of a plane.
Regrouping A process used when subtracting numbers that contain two or more digits and where one of the digits in the subtrahend is greater than the corresponding digit in the minuend; a "trading process" that utilizes the equivalents of 1 hundred for 10 tens or 1 ten for 10 ones, etc.
Regular Polygon A polygon in which all sides and all angles are congruent.
Regular Shape (See regular polygon)
Related Facts (See fact family)
Relative Error The ratio of the absolute error in a measurement to the size of the measurement; often written as a percent and called the percent of error; the absolute error is the difference between an approximate number and the true number that it approximates.
Relevant Information Information applicable to the problem. Information necessary for the solution of a problem. Data that is pertinent, applicable, and essential in the solution of a problem.
Remainder In whole number division, when you have divided as far as you can without using decimals, what has not been divided is the remainder. If the remainder is 0, there is no remainder.Example: When you divide 14 by four you get a remainder of 2.
Rename To express a number in a different form using regrouping or trading; used in performing arithmetic algorithms.
Repeated Addition Addition of equal groups; often used to model the concept of multiplication.
Repeated Subtraction Subtraction of equal groups from a number; a model or alternative algorithm for division.
Repeating Decimal A decimal in which one or more digits repeat infinitely. Example: 0.3333...or 0.3(with line over the 3). 5.27272...or 5.27 (with line over the 27).
Repeating Pattern A pattern of a group of items that repeats over and over.
Rhombus A four sided shape in which all of the sides are equal lengths. The polygon could be either a square or a parallelogram.
Right Angle An angle formed by two perpendicular lines, the measure of which is 90º.
Right Triangle A triangle that has one 90º angle (right angle).
Rigid Motion Rigid motion is a transformation of the plane or space, which preserves distance and angles.
Root Extraction Finding a number that can be used as a factor a given number of times to produce the original number; for example, the fifth root of 32 = 2 because 2 x 2 x 2 x 2 x 2 = 32).
Rotation The transformation that occurs when a figure is turned a certain angle and direction around a fixed point. It is also known as a "turn."
Rotational Symmetry A figure has rotational symmetry when it can be rotated around a central point, or point of rotation less than 360° and still be identical to the original figure.
Rounding Numbers When you divide 14 by four you get a remainder of 2.
Ruler A tool used to measure length.
Rules of Divisibility Patterns that make it easier to tell whether one number is divisible by another.


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