错误做法:

Measuring angles/lengths from diagrams labelled 'not to scale'

原因:

Exam diagrams are never drawn to scale, so measurements will give incorrect answers, and you will lose all reasoning marks.

正确做法:

Only use circle theorems and given values to calculate missing values, and always quote your reasoning.

错误做法:

Assuming any radius and tangent form a right angle, not just at the point of contact

原因:

The perpendicular rule only applies where the tangent touches the circle at its single point of contact.

正确做法:

Only apply the 90° angle rule for tangents and radii at the exact point of contact.

错误做法:

(Higher) Using addition instead of multiplication for intersecting chord properties

原因:

The rule uses products of segments, not sums, leading to incorrect length calculations.

正确做法:

Always multiply the lengths of the two segments of each chord when applying intersecting chord rules.

错误做法:

(Higher) Applying cyclic quadrilateral rules to non-cyclic quadrilaterals

原因:

The opposite angles sum to 180° rule only applies if all four vertices lie on the circumference of the circle.

正确做法:

Confirm the quadrilateral is explicitly labelled as cyclic, or all vertices lie on the circle, before using the cyclic quadrilateral angle rule.

错误做法:

Quoting vague reasoning like 'circle rule' instead of the specific theorem name

原因:

Exam answers require explicit reasoning to award full marks, vague statements get zero reasoning marks.

正确做法:

Always name the exact theorem you used, e.g. 'angle in a semicircle is 90°' instead of 'circle angle rule'.