Term

Definition

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 = x^{2} 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. 
rectilinear 
motion in a straight line 
recurring decimal 
decimal with a digit, or series of digits repeated infinitely. ^{1}/_{3} = 0.333333333333…… 
reentrant 
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 section 
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. 