# R

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random number generators
A device used to produce a selection of numbers in a fair manner, in no particular order and with no favor being given to any numbers. Examples include dice, spinners, coins, and computer programs designed to randomly pick numbers

range
The range of a set of numbers is the largest value in the set minus the smallest value in the set. Note that the range is a single number, not many numbers

range of the function f
The set of all the numbers f(x) for x in the domain of f

ratio
A rational number of the form a/b where a is called the numerator and b is called the denominator

ray
A straight line that begins at a point and continues outward in one direction

real numbers
Real numbers can be thought of as all the points falling along the number line in the coordinate plane

rectangle
A parallelogram with four right angles

recursion
Given some starting information and a rule for how to use it to get new information, the rule is then repeated using the new information

reflect
To perform a reflection

reflection
In the plane, a reflection is a rigid motion that keeps all the points on some line fixed, and flips the rest of the points to the opposite side of that line. In space, a reflection is a rigid motion that keeps all the points on one plane fixed, and flips all points to the opposite side of that plane. Note that if you perform any reflection twice, all points end up back where they started. When you reflect an object, you are creating a "mirror image" of that object, with the fixed line or plane being the mirror.

regular fractals
see fractal

regular polygon
A polygon whose side lengths are all the same and whose interior angle measures are all the same

relative frequency
Relative frequency is the number of items of a certain type divided by the number of all the numbers being considered

remainders
After dividing one number by another, if any amount is left that does not divide evenly, that amount is called the remainder. For example, when 8 is divided by 3, three goes in to eight twice (making 6), and the remainder is 2. When dividing 9 by 3, there is no remainder, because 3 goes in to 9 exactly 3 times, with nothing left over

residual
The observed value minus the predicted value. It is the difference of the results obtained by observation, and by computation from a formula.

rhombus
A parallelogram with four congruent sides

right angle
An angle of 90 degrees

right triangle
A triangle containing an angle of 90 degrees

rigid motion
A rigid motion, of the plane or of space, is one that keeps the distances between all pairs of points unchanged. Rotations, reflections and translations are examples of rigid motions.

Rose Curve
The graph of a function in polar coordinates of the form a*sin(b*t) or a*cos(b*t) where a ≠ 0 and b is an integer > 1

rotate
To perform a rotation

rotation
A rotation in the plane is a rigid motion keeping exactly one point fixed, called the "center" of the rotation. Since distances are unchanged, all the other points can be thought of as having moved on circles whose center is the center of the rotation. The "angle" of the rotation is how far around the circles the points travel. A rotation in three-dimensional space is a rigid motion that keeps the points on one line fixed, called the "axis" of the rotation, with the rest of the points moving some constant angle around circles centered on and perpendicular to the axis.