Geometry
What you must understand
Candidates must recognise and name lines, rays, angles and polygons, and classify angles as acute (less than 90°), right (90°), obtuse (between 90° and 180°) and reflex (greater than 180°). You need the angle facts: angles on a straight line total 180°, angles around a point total 360°, and vertically opposite angles are equal. Know the properties of triangles (angle sum 180°) and quadrilaterals (angle sum 360°), including squares, rectangles, parallelograms, rhombuses and trapeziums. Symmetry, nets of cubes and cuboids, and coordinates in the first quadrant also appear. Use a ruler and protractor accurately.
Traps to watch
- Angle sum errors: a triangle always totals 180°, never 360°.
- Naming shapes: a square is a special rectangle, and a rhombus has four equal sides but need not have right angles.
- Vertically opposite angles: they are equal only where two straight lines cross.
- Coordinates: the first number is across (x), the second is up (y). Reversing them is a common mistake.
- Nets: a cube net has six squares; check that faces do not overlap when folded.
Worked example
Two angles on a straight line are given as 115° and x. Find x.
Angles on a straight line add to 180°, so x = 180° − 115° = 65°.
Rule of thumb: before calculating an unknown angle, identify which angle fact applies - straight line (180°), point (360°), triangle (180°) or quadrilateral (360°).