Circle Properties
Edexcel International GCSE Mathematics A· 4.6· 25 min read
1. Key Circle Definitions (All Tiers)★☆☆☆☆⏱ 5 min
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All circle problems start with correctly identifying standard circle parts, as labelled in exam diagrams. All terms below are required recall for both Foundation and Higher tiers.
Standard Circle Parts
- Centre: Fixed point equidistant from all points on the circle
- Radius: Line segment from centre to any point on the circumference
- Diameter: Chord passing through the centre, length = 2 × radius
- Circumference: Perimeter of the circle
- Segment: Region bounded by a chord and an arc
Label the parts W, X, Y, Z on a circle diagram: W touches the circle at one point only, X is a region bounded by two radii and an arc, Y connects two points on the circumference, Z is a section of the circumference.
- 1
W is a tangent: it only touches the circle at one point
- 2
X is a sector: bounded by two radii and an arc
- 3
Y is a chord: connects two points on the circumference
- 4
Z is an arc: section of the circumference
2. Foundation Tier Chord & Tangent Properties★★☆☆☆⏱ 7 min
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These 3 properties are the only circle rules tested on Foundation tier, and are also used frequently in Higher tier problems. You must quote the exact property as reasoning to earn full marks.
Foundation Circle Properties
- Two tangents from the same external point are equal in length
- A tangent is perpendicular to the radius at the point of contact (90° angle)
- A perpendicular line from the centre to a chord bisects the chord (and vice versa)
A tangent of length 12 cm is drawn from point P to a circle with centre O and radius 5 cm. Point M is the midpoint of 8 cm long chord AB. Calculate (a) length OP, (b) length OM.
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Part (a): Tangent is perpendicular to radius at point of contact, so triangle OTP (T = point of contact) is right-angled.
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OP = √169 = 13 cm. Reasoning: Tangent perpendicular to radius at point of contact, Pythagoras' theorem.
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Part (b): Perpendicular from centre to chord bisects the chord, so AM = ½ AB = 4 cm.
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OM = 3 cm. Reasoning: Perpendicular from centre to chord bisects chord, Pythagoras' theorem.
Two tangents are drawn from point X to a circle, touching at Y and Z. If XY = 7 cm, what is XZ?
Reveal answer
7 cm —Correct: Tangents from the same external point are equal in length.
3. Higher Tier Intersecting Chord Properties★★★☆☆Higher only⏱ 5 min
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Higher tier students need to apply two intersecting chord rules, for chords that cross inside the circle, or lines that intersect outside the circle.
Intersecting Chord Properties
- Internal: If two chords intersect inside a circle, the product of the lengths of the segments of one chord equals the product of the segments of the other chord.
- External: If a tangent and secant intersect outside a circle, the square of the tangent length equals the product of the full secant length and its external segment.
Two chords AB and CD intersect inside a circle at point E. AE = 3 cm, EB = 8 cm, CE = 4 cm. Find length ED.
- 1
Apply internal intersecting chord property: AE × EB = CE × ED
- 2
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24 = 4 ED → ED = 6 cm. Reasoning: Product of segments of intersecting chords inside a circle are equal.
4. Higher Tier Circle Angle Theorems★★★★☆Higher only⏱ 8 min
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There are 5 core circle angle theorems for Higher tier, all required recall. You must name the exact theorem when giving reasoning for your answer.
Circle Angle Theorems
- Centre angle theorem: Angle at centre = 2 × angle at circumference for the same arc
- Semicircle angle theorem: Angle subtended by a diameter is 90°
- Same segment theorem: Angles subtended by the same arc in the same segment are equal
- Cyclic quadrilateral theorem: Opposite angles sum to 180°
- Alternate segment theorem: Angle between tangent and chord = angle in the alternate segment
In a circle with centre O, angle AOB = 100° (A and B on circumference). Point C is on the circumference in the opposite segment to O. (a) Find angle ACB. (b) If AB is a diameter, find angle ACB instead.
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Part (a): Apply centre angle theorem: angle at centre is twice angle at circumference.
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Reasoning: Angle subtended at centre is twice angle at circumference.
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Part (b): AB is diameter, so angle subtended by diameter is 90°.
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Angle ACB = 90°. Reasoning: Angle in a semicircle is a right angle.
5. Common Pitfalls
Wrong move:
Measuring angles/lengths from diagrams labelled 'not to scale'
Why:
Exam diagrams are never drawn to scale, so measurements will give incorrect answers, and you will lose all reasoning marks.
Correct move:
Only use circle theorems and given values to calculate missing values, and always quote your reasoning.
Wrong move:
Assuming any radius and tangent form a right angle, not just at the point of contact
Why:
The perpendicular rule only applies where the tangent touches the circle at its single point of contact.
Correct move:
Only apply the 90° angle rule for tangents and radii at the exact point of contact.
Wrong move:
(Higher) Using addition instead of multiplication for intersecting chord properties
Why:
The rule uses products of segments, not sums, leading to incorrect length calculations.
Correct move:
Always multiply the lengths of the two segments of each chord when applying intersecting chord rules.
Wrong move:
(Higher) Applying cyclic quadrilateral rules to non-cyclic quadrilaterals
Why:
The opposite angles sum to 180° rule only applies if all four vertices lie on the circumference of the circle.
Correct move:
Confirm the quadrilateral is explicitly labelled as cyclic, or all vertices lie on the circle, before using the cyclic quadrilateral angle rule.
Wrong move:
Quoting vague reasoning like 'circle rule' instead of the specific theorem name
Why:
Exam answers require explicit reasoning to award full marks, vague statements get zero reasoning marks.
Correct move:
Always name the exact theorem you used, e.g. 'angle in a semicircle is 90°' instead of 'circle angle rule'.
6. Quick Reference Cheatsheet
Tier | Property / Theorem | Rule | Exam Reasoning Phrase |
|---|---|---|---|
Foundation | Equal tangents | Two tangents from same external point are equal length | Tangents from a common external point are equal |
Foundation | Tangent-radius perpendicularity | Tangent ⊥ radius at point of contact | Tangent is perpendicular to radius at point of contact |
Foundation | Chord bisector rule | Perpendicular from centre to chord bisects chord | Perpendicular from centre of circle to chord bisects the chord |
Higher | Internal intersecting chords | AE × EB = CE × ED for intersecting chords AB, CD at E | Product of segments of intersecting chords are equal |
Higher | Centre angle theorem | Angle at centre = 2 × angle at circumference | Angle subtended at centre is twice angle at circumference |
Higher | Semicircle angle theorem | Angle subtended by diameter = 90° | Angle in a semicircle is a right angle |
Higher | Same segment theorem | Angles in same segment equal | Angles subtended by same arc in same segment are equal |
Higher | Cyclic quadrilateral theorem | Opposite angles sum to 180° | Opposite angles of a cyclic quadrilateral sum to 180° |
Higher | Alternate segment theorem | Angle between tangent and chord = angle in alternate segment | Alternate segment theorem |
7. Frequently Asked
Do I need to prove circle theorems in the exam?
No, formal proof of circle theorems is not required for Edexcel IGCSE Maths A. You only need to recall, apply them, and state the theorem name as reasoning for each step.
Which circle properties are tested on Foundation tier?
Foundation tier only covers circle part definitions and 3 chord/tangent properties: equal tangents from a point, tangent perpendicular to radius, and perpendicular from centre bisects a chord. All angle theorems and cyclic quadrilaterals are Higher-only.
Going deeper
What's Next
Once you have mastered all circle properties covered in this guide, you are ready to apply them to multi-step geometry problems and mixed topic exam questions. Circle properties are frequently combined with triangle rules (Pythagoras, trigonometry, congruence) and angle facts to make up 5-8 mark extended response questions on both Foundation and Higher tier papers. You should also practice past paper questions to familiarise yourself with common diagram layouts and reasoning phrasing expected by Edexcel examiners. Next, move on to sector and arc mensuration (including perimeter and area calculations for sectors and segments) to complete the geometry unit for your IGCSE Maths exam.