Three parallel-line angle facts
Corresponding angles are equal (F-shape), alternate angles are equal (Z-shape), and co-interior (allied) angles add to 180° (C-shape).
Exterior angles of any polygon sum to 360°
So for a regular polygon, exterior angle and interior angle exterior. Interior-angle sum .
Drawn from real examiner reports.
Mixing up interior and exterior angles
Students confuse the two and divide when they meant the interior, or vice versa.
Mislabelling angles on the diagram
Assigning a value to the wrong angle (e.g. deciding an interior angle is the exterior one) loses every mark even when the method is right.
Not spotting an isosceles triangle
After finding one angle, students fail to notice two equal sides/angles (e.g. radii or a symmetric shape) that unlock the next step.
Write each angle on the diagram with its reason
Marking angles on the figure (and naming the rule) earns method marks and prevents mislabelling.
Also basics: angles on a straight line sum to , angles at a point sum to , vertically opposite angles are equal.
Co-interior (allied) angles — what do they do?
Two straight lines are parallel. A transversal crosses them, and one pair of co-interior (allied) angles is marked. One of them is . Find the other.
