Reflection SymmetryA 2-D shape has reflection symmetry about a line if an identical-looking object in the same position is produced by reflection in that line. Example: In the shape ABCDEF, the mirror line runs through B and E. The part shape BCDE is a reflection of BAFE. Point A reflects onto C and F onto D. The mirror line is the perpendicular bisector of AC and of FD. |
Reflex AngleAn angle that is greater than 180 degrees but less than 360 degrees. |
Regular1. Describing a polygon, having all sides equal and all internal angles equal. 2. Describing a tessellation, using only one kind of regular polygon. Examples: squares, equilateral triangles and regular hexagons all produce regular tessellations. |
Relation, RelationshipA common property or connection between two or more variables. Example: in a linear graph of the form y = 2x, there is a linear relationship between x and y. For every x, y is half the size. Compare with 'correlation'. |
RemainderIn the context of division requiring a whole number answer (quotient), the amount remaining after the operation. Example: 29 divided by 7 = 4 remainder 1. |
ResultantA vector that is equivalent to the vector sum of two or more vectors. |
RhombusA parallelogram with all sides equal. |
Right AngleOne quarter of a complete turn. An angle of 90 degrees. An acute angle is less than one right angle. An obtuse angle is greater than one right angle but less than two. A reflex angle is greater than two right angles. Sometimes shortened to 'right' and used as an adjective, e.g. 'in a right cylinder the centre of one circular base lies directly over centre of the other'. |
RotationIn 2D, a transformation of the whole plane which turns about a fixed point, the centre of rotation. A is specified by a centre and an (anticlockwise) angle. |
Rotation SymmetryA 2D shape has rotation symmetry about a point if an identical-looking shape in the same position is produced by a rotation through some angle greater than 0 degrees and less than 360 degrees. A 2D shape with rotation symmetry has rotation symmetry of order n when n is the largest positive integer for which a rotation of 360/n degrees produces an identical-looking shape in the same position. |