Radian
A unit of angle measure. One radian is the angle at the center of a circle that cuts off an arc equal in length to the radius, about 57.3°.
Radical
An expression with a root sign √, such as √25 or ∛8. The sign itself is also called a radical.
Radicand
The number or expression under a radical sign. In √25, the radicand is 25.
Radius
A segment from the center of a circle to any point on the circle, or its length. It is half the diameter.
Random sample
A sample chosen so that every member of the population has an equal chance of being picked.
Range (data)
The difference between the greatest and least values in a data set. It is a simple measure of spread.
Range of a function
The set of all output values (y-values) of a function.
Rate
A ratio that compares two quantities with different units, such as 60 km per hour. In percent problems, the rate is the percent.
Rate of change
How much one quantity changes compared with another, such as the change in y divided by the change in x. For a line, it is the slope.
Rate of commission
The percent of sales paid to a salesperson as commission.
Rate of interest
The percent of the principal charged for borrowing money or earned on savings, usually per year.
Ratio
A comparison of two quantities by division, written as 3 : 2, 3 to 2 or 3/2.
Rational expression
A fraction whose numerator and denominator are polynomials, such as (x + 1)/(x − 2).
Rational number
Any number that can be written as a fraction a/b of two integers with b not 0, such as 3/4, −2 or 0.5.
Rationalize the denominator
To rewrite a fraction so that no radical is left in the denominator, by multiplying top and bottom by a suitable radical.
Ray
Part of a line that starts at one endpoint and goes on forever in one direction.
Real distance
The actual distance between two places, as opposed to the distance measured on a map.
Real numbers
All rational and irrational numbers: every number that has a place on the number line.
Reciprocal
The number you multiply by to get 1. The reciprocal of 3/4 is 4/3, and the reciprocal of 5 is 1/5.
Rectangle
A parallelogram with four right angles. Its opposite sides are equal.
Rectangular prism
A prism whose six faces are all rectangles, like a box; also called a cuboid.
Recursive formula
A formula that gives each term of a sequence from the term or terms before it, such as aₙ = aₙ₋₁ + 4 with a₁ = 3.
Reference angle
The acute angle between the terminal side of an angle in standard position and the x-axis.
Reflection
A transformation that flips a figure over a line, making a mirror image.
Reflex angle
An angle that measures more than 180° and less than 360°.
Reflexive property
Any quantity is equal to itself: a = a. In geometry, any figure is congruent to itself.
Regrouping
Exchanging 10 ones for 1 ten (or 1 ten for 10 ones, and so on) when adding or subtracting; also called carrying or borrowing.
Regular polygon
A polygon whose sides are all equal and whose angles are all equal.
Relation
Any set of ordered pairs. A function is a special relation in which each input has only one output.
Relative frequency
The number of times an outcome happens divided by the total number of trials or data values.
Remainder
The amount left over when one number does not divide another evenly.
Repeating decimal
A decimal in which one digit or a group of digits repeats forever, such as 0.333… or 0.1414…
Retail price
The price a store charges customers for an item, usually the wholesale cost plus a markup.
Rhombus
A parallelogram with four equal sides. Its opposite angles are equal and its diagonals are perpendicular.
Right angle
An angle that measures exactly 90°, marked with a small square.
Right triangle
A triangle with one right angle. The side opposite it is the hypotenuse.
Rigid transformation
A transformation that moves a figure without changing its size or shape: a translation, rotation or reflection.
Roman numerals
An ancient number system that uses letters: I = 1, V = 5, X = 10, L = 50, C = 100, D = 500 and M = 1000.
Root (of an equation)
A solution of an equation. For a function, the x-values where its graph crosses the x-axis; also called a zero.
Rotation
A transformation that turns a figure a set angle around a fixed point, the center of rotation.
Rotational symmetry
A figure has rotational symmetry if it looks the same after a turn of less than 360° around its center.
Rounding
Replacing a number with a nearby, simpler value, such as 47 rounded to the nearest ten is 50.
Row
A horizontal arrangement of items, such as numbers in a table, matrix or array.