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A Level · Cambridge (CIE) · 9709

Use circle symmetry, chords and diameters in coordinate problems

Combine a perpendicular chord bisector, Pythagoras and the angle in a semicircle with algebraic checks.

Pure Mathematics 1 · Coordinate geometry · 1.3

Pure Mathematics 1 (Paper 1) · AS & A Level Mathematics 9709, 2026–2027. Use algebra and elementary geometry; implicit differentiation is outside this section. Written practice is self-checked.

Before you begin

  • Find midpoints, exact distances and a circle's centre.
  • Use Pythagoras and identify perpendicular lines.

Work on paper. Try each question before revealing help, and explain why your method works.

Quick readiness check

What is the perpendicular distance from (2, −1) to the horizontal line y = 2?

What you will learn

  • Use circle symmetry to locate a chord midpoint and calculate its length.
  • Use diameter endpoints and the right angle in a semicircle to solve coordinate problems.
A perpendicular bisects a chord-4-202468-6-4-2024xyC(2, −1)A(−2, 2)B(6, 2)M(2, 2)345Full chord: 2 × 4 = 8
The circle (x − 2)² + (y + 1)² = 25 has a horizontal chord y = 2 with endpoints A(−2, 2) and B(6, 2). The perpendicular from centre C(2, −1) reaches midpoint M(2, 2), a distance 3 away. The radius CB is 5, so the right triangle CMB has half-chord MB = 4 and full chord AB = 8. The small square marks the right angle at M. Both axes use equal unit lengths.

A centre-to-chord perpendicular halves the chord

The perpendicular from a circle's centre to a chord passes through the chord midpoint. With radius r and perpendicular centre-to-chord distance d, a right triangle has half-chord length √(r² − d²), so the full chord length is 2√(r² − d²). This requires 0 ≤ d < r for a chord with two distinct endpoints. The equality d = r gives a tangent point rather than a positive-length chord.

For centre (2, −1), radius 5 and horizontal chord y = 2, the perpendicular meets the chord at (2, 2), so d = 3. The half-chord is √(25 − 9) = 4. The endpoints are (−2, 2) and (6, 2), and the full length is 8. Direct substitution into (x − 2)² + (y + 1)² = 25 verifies both endpoints.

A diameter determines a right angle at the circumference

If A and B are diameter endpoints and P is another point on their circle, angle APB is 90°. It is the angle at P, not the angle at the centre, and P must be distinct from A and B. Use the diameter midpoint as centre and half its length as radius to verify whether a proposed P is on that circle.

For A(−2, 1) and B(4, 5), the circle is (x − 1)² + (y − 3)² = 13. Point P(4, 1) lies on it because 3² + (−2)² = 13. PA is horizontal and PB vertical, confirming the semicircle right angle without dividing by an undefined gradient. Their lengths 6 and 4 also give PA² + PB² = 52 = AB².

Pause and explain

A radius is 5 and the centre-to-chord perpendicular distance is 3. What is the full chord length?

Put the idea to work

Worked example

For (x − 2)² + (y + 1)² = 25, find the endpoints, midpoint and length of the chord on y = 2.

Show the worked solution
  1. The centre is (2, −1), so the perpendicular reaches y = 2 at midpoint (2, 2), a distance 3 from the centre.
  2. The half-chord length is √(5² − 3²) = 4, giving x = 2 − 4 or 2 + 4.
  3. The endpoints are (−2, 2) and (6, 2); their midpoint is (2, 2) and their separation is 8.

Answer Endpoints (−2, 2), (6, 2); midpoint (2, 2); chord length 8.

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From guided practice to a new situation

Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.

1 · Guided

A circle has radius 5. A chord is a perpendicular distance 3 from its centre. Find its length.

Give me a hint

Use the right triangle formed by the radius and half the chord.

Compare my reasoning
  1. The radius is the hypotenuse and the perpendicular distance is one shorter side.
  2. Pythagoras gives half-length √(5² − 3²) = 4.
  3. Symmetry gives two equal halves, so the chord length is 8.

8.

Look for these in your work

  • Use the radius as the hypotenuse.
  • Double the computed half-length.
2 · Independent

Find the endpoints and length of the chord y = 6 on x² + y² = 100.

Give me a hint

Substitute the chord height into the circle equation.

Compare my reasoning
  1. The equation becomes x² + 36 = 100, so x² = 64.
  2. The endpoints are (−8, 6) and (8, 6).
  3. Their horizontal separation is 16; the midpoint is (0, 6), consistent with the circle's symmetry.

Endpoints (−8, 6), (8, 6); length 16.

Look for these in your work

  • Include both signs for the endpoint inputs.
  • Find full separation rather than the distance from midpoint to one end.
3 · Transfer

A circle has diameter endpoints A(−2, 1) and B(4, 5). Verify that P(4, 1) lies on it and explain angle APB.

Give me a hint

Find the diameter midpoint and squared radius, then use the semicircle theorem or perpendicular directions.

Compare my reasoning
  1. The centre is (1, 3) and r² = (6² + 4²)/4 = 13.
  2. For P, the squared centre distance is (4 − 1)² + (1 − 3)² = 13, so P lies on the circle and differs from both endpoints.
  3. Because AB is a diameter, angle APB is 90°; PA horizontal and PB vertical independently confirm it.

P lies on (x − 1)² + (y − 3)² = 13; angle APB = 90°.

Look for these in your work

  • Verify the point in the actual diameter circle.
  • Identify the right angle at the circumference point P.

Common mistakes

  • The radius is the hypotenuse of the half-chord triangle.
  • Double the half-chord length to get the full chord.
  • An angle in a semicircle is at a circumference point distinct from the diameter endpoints.
Recall without your notes

What can you explain now?

A circle has radius 13 and a chord whose perpendicular distance from the centre is 5. Find the chord length.

Compare with the explanation

24.

Its half-length is √(169 − 25) = 12, so the full chord is 24.

After trying it yourself, choose your next review. This is your self-assessment.

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Make it stick

Explain the two different midpoint ideas in a diameter and a chord. Sketch the half-chord right triangle and locate the angle in a semicircle.

If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.

Keep learning Mathematics

Course scope and assessment

312 sequenced lessons provide 936 guided, independent and application problems covering the 160 objectives in sections 1.1–1.8, 2.1–2.6, 3.1–3.9, 4.1–4.5, 5.1–5.5 and 6.1–6.5. The map lists all 38 sections across six alternative components; all eight Pure Mathematics 1 sections and all six Pure Mathematics 2 sections, from Paper 2 Algebra through Numerical solution of equations, currently have full teaching sequences. Paper 3 Algebra, section 3.1, links the six shared foundation lessons plus six lessons on partial fractions and rational binomial expansions; Paper 3 logarithms, trigonometry and numerical equations use the shared lessons, with the Paper 1 graph foundation linked into trigonometry. Paper 3 differentiation combines the shared methods with its inverse-tangent derivative lesson. Paper 3 Integration links four shared standard and identity lessons plus seven advanced lessons, covering all six section 3.5 requirements. Eight Vectors lessons cover all six section 3.7 requirements, with 2D and 3D line and solid applications. Nine Differential equations lessons cover all four section 3.8 requirements, including Paper 3 integration techniques, initial conditions, equilibrium solutions and conditional model interpretations. Eleven Complex numbers lessons cover all eight section 3.9 requirements, with seven Argand diagrams and explicit argument and boundary checks. All nine Pure Mathematics 3 sections now have teaching linked to every mapped requirement. Nine Forces and equilibrium lessons cover all seven section 4.1 requirements, with six force diagrams, signed incline-friction intervals, reaction-dependent pull and push limits and separate-body third-law checks. Nine Kinematics lessons cover the four official section 4.2 requirements, with the graph requirement mapped to three relationships. Six motion diagrams support directed quantities, graph slopes and signed areas, Paper 1 time-calculus methods, reversal distances, constant-acceleration stages and shared-clock particle meetings. Six Momentum lessons cover both section 4.3 requirements, including signed system totals, conservation assumptions, outgoing velocities, coalescence, unknown masses and successive impacts. Four diagrams show system momentum through the stipulated events, and supplied kinetic-energy checks distinguish conservation of momentum from changes in kinetic energy. Twelve Newton’s laws lessons cover all four section 4.4 requirements, with vertical support and release stages, rough horizontal and incline motion, inferred parameters, hanging and surface pulley systems, rigid tow-bar thrust and slack-string transitions. Seven force diagrams support the separate-particle balances. Nine Energy, work and power lessons cover all five section 4.5 requirements, with angle-dependent work, kinetic and reference potential energy, signed net-work balances, smooth curved slides, rough and driven slopes, ideal connected-system energy and instantaneous power on hills. Five diagrams distinguish the energy inputs, losses and valid positive-speed force model. All five Mechanics sections now have teaching and staged practice linked to their mapped requirements. Thirteen Representation of data lessons cover all five section 5.1 requirements, including choice and limitations of displays, ordered and back-to-back stems, raw summaries and box plots, unequal-width density histograms, cumulative-frequency interpolation, descriptive moment calculations, grouped estimates, coding, combining two datasets and correcting totals. Six accessible diagrams retain repeated observations and distinguish numerical spread, frequency area and estimated readings. Ten Permutations and combinations lessons cover both official section 5.2 requirements, with selection and ordered roles, repeated objects, adjacency blocks, non-adjacency gaps, leading-zero cases, labelled multi-row seating and mixed restrictions. Six accessible diagrams show outcome trees, ordered versus unordered choices, repeated copies, blocks, gaps and distinct row seats. Eleven Probability lessons cover all four section 5.3 requirements, with equiprobable elementary outcomes, selection-based probabilities, complements and disjoint regions, independence tests, restricted sample spaces, two-way tables, replacement and removal trees, reversed conditions and unknown parameters. Seven accessible diagrams show dice outcomes, random subsets, event regions, a conditioned grid, a count table and two probability trees. Twelve Discrete random variables lessons cover all three official section 5.4 requirements, with complete probability tables, moment and parameter checks, binomial assumptions, exact probabilities and moments, geometric first-success probabilities, tails, expectation and integer reliability thresholds. Six accessible diagrams distinguish outcome grouping, weighted moments, fixed trials, discrete probability bars, waiting conventions and integer tail deadlines. Twelve Normal distribution lessons cover all three official section 5.5 requirements, with continuous-area interpretation, MF19 cumulative tables, full standardisation, tail and interval regions, conditional probabilities, percentiles, one- and two-parameter inference, binomial suitability, continuity corrections and approximate integer thresholds. Seven accessible diagrams show model areas, table reading, aligned scales, intervals, quantiles, matched binomial and normal shapes and whole integer cells. All five Statistics 1 sections now have teaching and staged practice linked to their mapped requirements. Fourteen Poisson distribution lessons cover all five official section 6.1 requirements, with homogeneous event-count assumptions, exposure scaling, exact masses and tails, conditional count events, moments, parameter inference, probability duration targets, disjoint and overlapping windows, rare-count binomial approximations, matched normal approximations and continuity corrections. Nine accessible diagrams show event exposures, unit conversions, discrete bars, complete tail complements, population moments, shared windows, approximate count models and integer cells. Exact checks distinguish approximate and exact close-target decisions. Fourteen Linear combinations lessons cover the six listed results in official section 6.2, with affine moments, signed scales and constant cases, expectation without independence, independent squared variance contributions, fixed and random fees, independent copies versus reused values, affine normal events, independent normal sums and differences, weighted and conditional areas, coefficient inference, exact independent Poisson sums and overlapping exposure limits. Nine accessible diagrams distinguish scale changes, variance contributions, component costs, copy mechanisms, aligned normal scales, summed areas, Poisson count pairs and unsupported distribution claims. Fourteen Continuous random variables lessons cover both official section 6.3 requirements, with density validity and units, support-aware intervals, complements and conditional regions, first and second moments, percentiles by direct density areas, symmetry checks, parameter inference, infinite-support power and exponential integrals, divergent-moment limits and combined applications. Nine accessible diagrams distinguish density height from probability, normalisation, negative-density failures, central and conditional areas, mean versus median, percentile locations and partial infinite tails. Explicit cumulative distribution function knowledge is not required. Eighteen Sampling and estimation lessons cover all eight official section 6.4 requirements, with population and frame definitions, random-number selection, sampling-quality limits, independent sample-mean moments, normal means, CLT conditions, SE scaling, raw, summarised and coded unbiased estimates, known-variance and large-sample mean intervals, repeated-sampling interpretation, precision planning, proportion intervals, sparse-outcome limits and contextual applications. Ten accessible diagrams distinguish selection frames, number rejection, repeated sample means, population versus mean density, CLT models, variance divisors, interval targets, illustrative coverage, integer size checks and proportion margins. Eighteen Hypothesis tests lessons cover all five official section 6.5 requirements, with pre-specified alternatives, inclusive count tails, explicit equal-tail conventions, integer critical regions and actual significance, exposure-aware Poisson tests, continuity-corrected count approximations, normal and large-sample mean tests, confidence-interval connections, error classifications, fixed-region binomial, Poisson and normal error probabilities and sample-size power comparisons. Nine accessible diagrams show the testing procedure, exact count regions, unequal Poisson tails, discrete significance jumps, corrected count boundaries, matching mean intervals and alternative error distributions. All shared lessons are counted once in the course total. All 38 sections now link teaching and practice. Eight earlier overview lessons remain available, but do not count as full section coverage. This is a content-availability measure, not full programme completion or exam readiness. Written practice is self-checked, not officially marked. Check the 2028–2030 syllabus separately if sitting an examination from 2028; this teaching map uses 2026–2027 requirements.

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