Radial symmetry - .

Radian - .

Radius - The distance from the center of a circle to its edge. Half a diameter. See circumference.

Ratio - A comparison of two quantities, often separated by a colon (:). Can be expressed as a fraction. See proportion.

Rational number - Any number which can be expressed as a fraction of two integers. In decimal form, either terminates or repeats. Includes the natural numbers and integers. See Real number.

Rationalize - .

Ray - A portion of a line, infinite in length and terminated at one end by an endpoint. The other end does not terminate. See Line segment.

Real number - Any number corresponding to a point on the number line. Includes the natural numbers, integers, rational numbers and irrational numbers. Any real number which is not rational is irrational.

Reason - Justifies a statement in a proof.

Reciprocal - Multiplicative inverse.

Rectangle - .

Reflection See line reflection, isometry.

Region - .

Regular - .

Relatively prime numbers - Numbers having no factors in common. Note that a number by itself cannot be relatively prime - it must be compared with another number. Prime numbers are always relatively prime to any other natural number greater than one, including other primes.

Remainder - The result of a subtraction problem. A difference. What is left over. The amount left over after a subtraction within a division problem. Divisible numbers always have a remainder of zero.

Remote - .

Replacement set - .

Rhombus - .

Rhomboid - .

Right angle - A 90 degree angle. Formed by two perpendicular lines, rays, segments or plane surfaces.

Right triangle - A triangle with one right angle. Note that it is impossible for a triangle in a plane to have more than one right angle.

Root of an equation - A solution to an equation. A value of the independent variable for which the dependent variable is zero. A point on the graph of an equation which crosses the horizontal axis. An equation may have more than one root, or none. The maximum number of roots is equal to the order (highest power) of the equation. Equations with odd order always have at least one root.

Root of an expression - An inverse power. A value or expression raised to the corresponding power (used as a factor that many times) equals the expression. For even-numbered powers, there is a negative solution for each positive solution and having the same magnitude.

Rotation - A change in the attitude (angle of orientation) of a figure around a point (center of rotation). One of the similarity transformations (see translation, line reflection or isometry, dilation).

Round - .

Rounding - .

Ruler - A straightedge with regular markings used to measure Ruler postulate - .

Last updated 6/2/97 - 4/28/2000.