radius of a circle or sphere A segment connecting the center of a circle or a sphere with a point on that circle or sphere, also, the length of that segment. Plural is radii.
Ex: miles per hour, beats per minute, candy bars for each child
Rate Model for Division If a and b are quantities with different units, then a/b is the amount of quantity a per quantity b.
Rate Factor Model for Multiplication When a rate is multiplied by another quantity, the unit of the product is the ( product ) of units multiplied like fractions. The product has meaning whenever its units have meaning.
Ex: You can see that the following rate multiplication changes 6 days into its equivalent number of seconds:
Ratio Comparison Model for Division If a and b are quantities with the same units, then a/b compares a to b.
ratio of similitude In similar figures, the ratio of a distance or length in an image to the corresponding distance or length in a preimage.
Ex: When comparing the following small right triangle to its similar large right triangle, we see a ratio of similitude of 0.5.
rational number A number that can be written as a simple fraction. These numbers can also be written as terminating or repeating decimals.
Ex:
ray A part of a line which begins at some point and goes on forever in a particular direction. The ray with endpoint A containing B is denoted 
.
. real numbers A number that can be written as a decimal. The set of Real numbers is made up of all rational numbers as well as all irrational numbers.
reciprocal A number which, when multiplied by a given number, yields the product 1. Also called multiplicative inverse. An example of this is the numbers 2 and 1/2. When multiplied, they yield 2/2, which equals 1.
reflecting line The line over which a preimage is reflected. Also called mirror line or line of reflection.
 | Here we see the classic heart shape. It has a mirror line drawn vertically through its center. |
reflection A transformation in which each point is mapped onto its reflection image over a line or plane.
region The union of a polygon or circle with its interior is known as a region.
regular polygon A convex polygon whose sides all have the same length and angles all have the same measure.
regular polyhedron A convex polyhedron whose faces are all congruent regular polygons and the same number of edges intersect at each of its vertices. There are only five of these that can be constructed out of a single polygon shape. They are:
 |  |
regular pyramid A right pyramid whose base is a regular polygon and whose lateral faces are congruent isosceles triangles.
 | Here we see a right pyramid with a square base. It would be classified as a regular pyramid. |
Related Facts Property of Multiplication and Division If xy = P, then P/x = y and P/y = x. Or a numerical example:
If you know that 4x6=24, then the related facts are 24/4=6 and 24/6=4.
If you know that 4x6=24, then the related facts are 24/4=6 and 24/6=4.
Repeated Addition Model for Multiplication If n is a positive integer, then nx = x + x + . . . + x. ( n addends)
Ex:"16" is the repetend in this example.
right angle An angle whose measure is 90°. A right angle will always be drawn with a small square at its vertex. This removes all doubt that it is a 90 degree angle. If you don't see the square you cannot assume that it is a right angle.
right angle method A method for finding the center of a circle that involves constructing two inscribed right triangles. The intersection of their hypotenuses is the center.
right cylinder A cylinder formed when the two circular bases are directly across from each other. The walls of a right cylinder are perpendicular to both bases.
right prism A right prism is formed when two polygonal bases are placed right across from each other. The walls of a right prism are perpendicular to both bases.
right pyramid A pyramid whose base is a regular polygon and in which the segment connecting its vertex to the center of its base is perpendicular to the plane of the base.
 | In a right pyramid, the vertex will always be directly above the center of the regular polygon base. |
 | In this example we see a right triangle with legs a and b and hypotenuse c. We also see an equation known as the Pythagorean Theorem which is true for every right triangle. |
rotation A transformation in which every point of the preimage is rotated by a given angle about a point (in two dimensions) or a line (in three dimensions).
rotation-symmetric figure A figure which when rotated less than 360 degrees will lie on top of itself.
Ex: 13.74 rounded down to the preceding whole number is 13.
rounding to the nearest Making an estimate to a particular decimal place by either rounding up or rounding down depending on which estimate is closest to the exact value.
Ex: 13.74 rounded to the nearest whole number is 14.
Ex: 13.74 rounded up to the next tenth is 13.8.
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