The best spot for math. Mathematical "R" Words
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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.
Fom the center to the edge.

rate  A quantity whose unit contains the word " per " or  " for each " or some synonym.

Ex: miles per hour, beats per minute, candy bars for each child

rate factor A rate used in multiplication .

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. 

Notice the units divide out like common factors leaving you with an answer in seconds only.
Ex: You can see that the following rate multiplication changes 6 days into its equivalent number of seconds:

rate unit  The unit of measurement in a rate.

ratio  The quotient of two quantities with the same units.

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.
All the sides on the small figure are 1/2 the length of the large triangle. This can go the other way too, and be read as a ratio of 2 to 1.

rational number A number that can be written as a simple fraction. These numbers can also be written as terminating or repeating decimals.

Ex: These can be all written as simple fractions.

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 .
'A" is the endpoint of the ray, even though you might want to call it the "start" point.

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.

rectangle  A quadrilateral with four right angles.
Four sides and four right angles.

rectangular array  An arrangement of objects into rows and columns.
A matrix is an example of a rectangular array.

rectangular solid  A box. The union of a box and its interior.
A solid includes the skin and all the stuff inside.

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.

reflecting plane The plane over which a preimage is reflected. Also called mirror.

reflection A transformation in which each point is mapped onto its reflection image over a line or plane.

reflection image   The image of a figure reflected over a line.

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.
Same legth sides and same measure angles.

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:

  1. Tetrahedron (four faces each an equilateral triangle)
  2. Cube or Hexahedron (six faces each a square)
  3. Octahedron (eight faces each an equilateral triangle)
  4. Icosahedron (twenty faces each an equilateral triangle)
  5. Dodecahedron (twelve faces each a regular pentagon)

regular prism A right prism whose base is a regular polygon.
Here we see a regular triangular prism.

regular pyramid A right pyramid whose base is a regular polygon and whose lateral faces are congruent isosceles triangles.

Here we see a square base. 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.

Repeated Addition Model for Multiplication  If n is a positive integer, then nx = x + x + . . . + x. ( n addends)

repetend The digits which repeat forever in an infinitely repeating decimal.

Ex: "16" is the repetend in this example.

revolution  A turn of 360. Also called full turn.

rhombus  A quadrilateral with all sides of the same length.
A rhombus does not have to have right angles.

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.
Always look for the little square in the corner to be sure you have a right triangle.

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.
Draw two right triangles,  and where the hypotenuses cross is the center of the circle.

right cone A cone whose axis is perpendicular to the plane of the circular base.
Looks like and icecream cone.

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.
Like a soup can.

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.

right triangle A triangle with a right angle.

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).
These are also called turns.

rotation-symmetric figure A figure which when rotated less than 360 degrees will lie on top of itself.
Turn them less than 360 degrees and they lie on top of each other.

rounding down  Making an estimate that is smaller than the exact value.

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.

rounding up  Making an estimate that is bigger than the exact value.

Ex: 13.74 rounded up to the next tenth is 13.8.

round-robin tournament A tournament in which each competitor plays each other exactly once.

row  A horizontal line of objects in a rectangular array.

Rows and Columns.

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