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A Level · Cambridge (CIE) · 9709

Connect sectors, chords, triangles and circular segments

Use the sector's isosceles triangle to find a chord, separate a segment from its sector and calculate mixed boundaries.

Section 1.4 · Circular measure

Pure Mathematics 1 · Circular measure · 1.4

Pure Mathematics 1 (Paper 1) · AS & A Level Mathematics 9709, 2026–2027. Use radians in arc and sector formulae. Written practice is self-checked.

Before you begin

  • Use s = rθ and A = r²θ/2.
  • Use elementary sine and cosine with the correct angle mode.

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

Quick readiness check

Which straight boundary belongs to a circular segment?

What you will learn

  • Calculate chord lengths and triangle areas using a central angle.
  • Find minor segment areas, major complements and boundaries containing arcs and chords.
Segment = sector − triangleπ/36 cm6 cmArc 2π cmChord 6 cmOABSector: 6π · Triangle: 9√3 cm²Segment: 6π − 9√3 cm²
A circle of radius 6 cm has a minor central angle π/3. The blue sector contains its central triangle and the darker segment between chord AB and the arc. Sector area is 6π cm², triangle area is 9√3 cm² and minor segment area is 6π − 9√3 cm². The chord is 6 cm and the arc is 2π cm, so the segment perimeter is 6 + 2π cm. The two radius lengths and angle are drawn to scale.

Halve the isosceles triangle to find the chord

Join both arc endpoints to the centre. The two radii form an isosceles triangle with included angle θ. For 0 < θ < π, bisecting the angle produces a right triangle with hypotenuse r and opposite side half the chord. Therefore chord length c = 2r sin(θ/2), and the perpendicular centre-to-chord distance is r cos(θ/2). The arc length rθ and chord length are different.

The triangle area is (1/2)r² sin θ, using the two-side included-angle formula. For r = 6 cm and θ = π/3, the chord is 12 sin(π/6) = 6 cm and the triangle area is 18 sin(π/3) = 9√3 cm². The curved arc is 2π cm, slightly longer than the chord. Evaluate the sine in radian mode when the supplied angle is in radians.

A segment is the region between a chord and its arc

For a minor central angle 0 < θ < π, the minor segment area equals sector minus triangle: Asegment = r²(θ − sin θ)/2. Its perimeter is its arc plus its chord, not the two radii. With r = 6 and θ = π/3, the area is 6π − 9√3 cm² and the perimeter is 2π + 6 cm. Check that the segment area is positive and smaller than the sector area.

For a major segment bounded by the same chord, subtract the minor segment area from πr² and use the major arc angle 2π − θ for its boundary. Do not subtract an unsigned triangle area blindly from a sector with a reflex angle; identify the region first. Compound regions can be split into sectors and triangles, then added or subtracted without double-counting shared internal edges.

Pause and explain

For r = 6 and θ = π/3, what is the minor segment area?

Put the idea to work

Worked example

For a circle of radius 6 cm, a minor segment has central angle π/3. Find its exact area and perimeter.

Show the worked solution
  1. The sector area is 6²(π/3)/2 = 6π cm².
  2. The triangle area is 6² sin(π/3)/2 = 9√3 cm², so subtract it to get 6π − 9√3 cm².
  3. The chord is 2(6)sin(π/6) = 6 cm and the arc is 2π cm, so their sum is 6 + 2π cm.

Answer Area 6π − 9√3 cm²; perimeter 6 + 2π cm.

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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 minor segment has radius 4 cm and central angle π/2. Find its area and chord length.

Give me a hint

Calculate the sector and triangle separately, and halve the angle for the chord.

Compare my reasoning
  1. The sector area is 4²(π/2)/2 = 4π cm².
  2. The triangle area is 4² sin(π/2)/2 = 8 cm², giving segment area 4π − 8 cm².
  3. The chord is 2(4)sin(π/4) = 4√2 cm.

Area 4π − 8 cm²; chord 4√2 cm.

Look for these in your work

  • Subtract the correct triangle from the sector.
  • Use the half-angle when calculating a chord.
2 · Independent

A circle has radius 10 cm and a minor chord of length 10 cm. Find the minor central angle and segment area exactly.

Give me a hint

Use half the chord and radius in a right triangle, then determine the minor angle.

Compare my reasoning
  1. The half-chord relation gives sin(θ/2) = 10/(2 × 10) = 1/2.
  2. Because the minor angle is below π, θ/2 = π/6, hence θ = π/3.
  3. The segment area is 100(π/3 − sin(π/3))/2 = 50π/3 − 25√3 cm².

Angle π/3 radians; area 50π/3 − 25√3 cm².

Look for these in your work

  • Use the minor-angle restriction to choose the correct inverse value.
  • Retain exact sector and triangle terms.
3 · Transfer

A chord cuts a circle of radius 6 cm into minor and major segments. The minor angle is π/3. Find the major segment area and its perimeter exactly.

Give me a hint

Use the whole circle as an area complement, and the major arc with the same chord.

Compare my reasoning
  1. The minor segment area is 6π − 9√3, so the major area is 36π − (6π − 9√3) = 30π + 9√3 cm².
  2. The major arc angle is 2π − π/3 = 5π/3, so its length is 6(5π/3) = 10π cm.
  3. The unchanged chord has length 6 cm, giving major segment perimeter 10π + 6 cm.

Major area 30π + 9√3 cm²; perimeter 10π + 6 cm.

Look for these in your work

  • Subtract the minor area from the full circle.
  • Use the major arc and count the chord once.

Common mistakes

  • A segment is not a sector: its straight edge is the chord.
  • Use θ/2 for a half-chord triangle, but θ in the full triangle area.
  • Identify minor and major regions before choosing a subtraction.
Recall without your notes

What can you explain now?

A minor segment in a circle of radius 4 has angle π/2. Find its area.

Compare with the explanation

4π − 8 square units.

The sector area is 8(π/2) = 4π and the triangle area is 8 sin(π/2) = 8.

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

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Make it stick

Draw a sector, its isosceles triangle and its minor segment. Label which angle each sine uses and explain why the sector and segment have different perimeters.

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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