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A Level · Cambridge (CIE) · 9709

Radians and circular measure

Use radians to connect an angle directly to lengths and areas.

Pure Mathematics 1 · Circular measure revision · 1.4

Pure Mathematics 1 (Paper 1) · Mathematics 9709, 2026–2027. This short revision lesson connects angle units, sector boundaries and segment area; it does not replace the detailed section 1.4 pathway. Written solutions use self-check criteria, not an official mark scheme. Specialist accuracy review has not been completed.

Before you begin

  • Use circumference 2πr and area πr².
  • Rearrange simple formulae and use triangle area ½ab sin C with consistent angle units.

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

Quick readiness check

A circle has radius 8 cm. Which calculation gives the arc length for a central angle of 75°?

What you will learn

  • Convert degrees to radians.
  • Calculate arc length and sector area.
120°r = 6arc 4πOFull perimeter = 4π + 12
A 120° sector of radius 6 has arc length 4π and area 12π. The entire boundary includes two radii, so perimeter is 4π + 12. Sketch not to scale.

A radian is a length ratio

An angle of one radian subtends an arc equal in length to the circle's radius. Since a full circumference is 2πr, a full turn is 2π radians. Thus 180° corresponds to π radians.

The formula s=rθ works when θ is in radians. Degrees must be converted first. Keep exact multiples of π until the end if exact form is requested.

Sector area is a fraction of a circle

The sector occupies the fraction θ/(2π) of a full circle. Multiplying that by πr² gives 1/2 r²θ. If asked for a sector perimeter, include both radii as well as the arc.

s=rθ; sector area=½r²θ

Sources for this lesson

Pause and explain

A sector has radius 8 cm and angle 1.5 radians. A wire follows its entire boundary. How long is the wire?

Put the idea to work

Worked example

A sector has radius 9 cm and angle π/3 radians. Find its arc length and perimeter.

Show the worked solution
  1. Use s=rθ=9×π/3=3π cm.
  2. The boundary also includes two radii totalling 18 cm.
  3. Add the straight and curved parts.

Answer Arc length 3π cm; perimeter 18+3π cm

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

The diagram shows a sector with radius 6 and angle 120°. Taking the lengths in centimetres, derive its exact arc length, area and full perimeter. Explain why the two radii are not part of the arc length.

Give me a hint

Convert 120° to radians, then separate the curved boundary, straight boundary and enclosed area.

Compare my reasoning
  1. 120° × π/180 = 2π/3 radians. The arc length is 6 × 2π/3 = 4π cm.
  2. The sector area is ½ × 6² × 2π/3 = 12π cm². This is one third of the full circle area 36π.
  3. The full perimeter is 4π + 6 + 6 = 4π + 12 cm. The two radii are straight edges, whereas the arc is only the curved edge.

Arc: 4π cm; area: 12π cm²; perimeter: 4π + 12 cm.

Look for these in your work

  • Convert the angle before substitution.
  • Use squared length units only for area.
  • Count two radii for perimeter and none for arc length.
2 · Independent

A sector has arc length 15 cm and area 45 cm². Find its radius, central angle in radians and full perimeter. Check your answers in both given measurements.

Give me a hint

Eliminate θ from s = rθ and A = ½r²θ; do not confuse the given arc with the perimeter.

Compare my reasoning
  1. Since s = rθ, the area is A = ½rs. Thus r = 2A/s = 90/15 = 6 cm.
  2. The angle is θ = s/r = 15/6 = 2.5 radians. The full perimeter is 15 + 2(6) = 27 cm.
  3. Check: rθ = 6 × 2.5 = 15 cm and ½r²θ = ½ × 36 × 2.5 = 45 cm². Also 0 < 2.5 < 2π, consistent with a sector smaller than a whole circle.

Radius 6 cm; angle 2.5 radians; perimeter 27 cm.

Look for these in your work

  • Eliminate the same angle from both formulae.
  • Distinguish the known arc from the full boundary.
  • Substitute back into the original length and area.
3 · Transfer

A circle has radius 5 cm. A chord and its minor arc enclose a segment with central angle 2π/3 radians. Find the exact segment area and perimeter. Explain why subtracting the triangle changes both the area calculation and the straight boundary.

Give me a hint

Sketch two radii and the chord. Remove the isosceles triangle from the sector; use the chord, not two radii, in the segment perimeter.

Compare my reasoning
  1. The sector area is ½ × 25 × 2π/3 = 25π/3 cm². The triangle area is ½ × 25 × sin(2π/3) = 25√3/4 cm².
  2. Subtract the triangle: segment area = 25π/3 − 25√3/4 cm². It is positive and smaller than the sector area because 0 < 2π/3 < π.
  3. Bisecting the isosceles triangle gives chord length 2 × 5 × sin(π/3) = 5√3 cm. The minor arc is 5 × 2π/3 = 10π/3 cm.
  4. The segment perimeter is 10π/3 + 5√3 cm. The chord replaces the two radii as the straight part of this boundary; it is not added to them.

Area: 25π/3 − 25√3/4 cm²; perimeter: 10π/3 + 5√3 cm.

Look for these in your work

  • Identify the minor segment and subtract its central triangle.
  • Use the half-angle for the chord and full angle for triangle area.
  • Trace only the boundary of the requested region.

Common mistakes

  • Using degrees directly in s=rθ gives an incorrect result.
  • An arc length is not the entire sector perimeter.
Recall without your notes

What can you explain now?

Find the area of a sector of radius 4 cm and angle π/2.

Compare with the explanation

4π cm²

Use 1/2×16×π/2=4π.

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

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Make it stick

Without notes, explain why s = rθ needs radians. Sketch a sector and its minor segment, and label which straight edges belong to each perimeter. Then derive r = 2A/s for a sector from its area A and arc length s.

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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