radian unit for measuring angles. $$\pi^r$$= 180° and 2$$\pi$$ = 360°. $$1^r=\frac{180}{\pi}$$ and 1° =$$\frac{\pi}{180}$$
radius the name given to a line from the centre of a circle to the circumference. The radius is half the length of the diameter. The radius will also meet the tangent in a right angle.
random term in probability when each selection is just as likely as another. to choose randomly means that there is no reason one would be picked more than another.
range the uppermost score minus the lowermost score
range of function refers to the y values a function can take. For example the range of y = x2 is R: y ≥ 0
rate the amount one quantity changes with respect to another. Speed is the most commonly used rate, km/h measures how many kilometres are travelled over time. Rates are expressed in the terms of two units, unit1 per unit2, or such as words per minute, litres/minute
ratio a comparison between two amounts. Ratios are generally unit free, and whole numbers. An example of 50m:1km would be simplified to 50:1000 and then down to 1:20
rational number any number that can be expressed as a whole number, fraction, finite or recurring decimal
ray half a line. Starting from a point and extending in only one direction, instead of both

real number all rational and irrational numbers
reciprocal the upside down fraction, or when 1 is divided by the number. for example the reciprocal of 7/9 is 9/7 or the reciprocal of 2 is 1/2, the reciprocal of a/b is b/a
rectangle quadrilateral with opposite sides equal and parallel. Diagonals are equal and bisect each other. Adjacent sides perpendicular

rectangular numbers all numbers that have more than two factors and can be represented by dots forming a rectangle. For example 6 is a rectangular number. The first rectangular number is 2 × 1 = 2, the second would be 3 × 2 = 6, the third would be 4 × 3 = 12 and so on.

rectangular prism solid shape made up of six rectangles, with a rectangular cross section. A box. r-prism
rectilinear motion in a straight line
recurring decimal decimal with a digit, or series of digits repeated infinitely. 1/3 = 0.333333333333……
re-entrant polygon with one angle more than 180°, also known as a concave polygon
reflection transformation that ‘flips’ a shape in the line of reflection
reflex angle angle more than 180° but less than 360°
region area of a graph that is defined by an inequality
regular polygon polygon with all equal sides and equal angles
relative frequency the proportion of times data occurs in a distribution. The relative frequency is obtained by dividing how many times that score occurs by the total number of scores
remainder how much is left over when something is divided and it does not fit evenly
remainder theorem when P(x) is divided by (x – a), then the remainder will be ƒ(a)
revolution angle at a point, full turn angle, 360°
rhombic prism a prism with a rhombus as a cross sectionrhombicprism
rhombus a quadrilateral (4 sides) with opposite sides parallel and all sides equal. Also called a ‘diamond’ or a ‘squishy square’. Diagonals bisect at right angles.
right angle angle formed when two lines meet at 90°, or are perpendicular. Right angles are indicated with a little square.

right pyramid pyramid with its vertex directly over the centre of the base  

roman numerals number system the Romans used to use. l = 1, V = 5, X = 10, L = 50, C = 100, D = 500, M = 1000, V = 5000, X = 10000, L = 50000, C = 100000, D = 500000, M = 1000000
root the solution to an equation.
root shortened version of square root
rotate to turn
rotation the act of turning
rotation transformation where a shape is moved around a fixed point
rotational symmetry when a shape can be rotated so that it fits exactly onto its own outline more than once in a full turn
rounding off writing a number correct to a specified number of decimal places. See Rounding page for fully worked examples and explanations.
rpm abbreviation for revolutions per minute. How many time something turns around completely in one minute.

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