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A Level · Cambridge (CIE) · 9709

Define a radian and convert angle units accurately

Measure an angle by arc length divided by radius, convert exact angles and select the matching calculator mode.

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 circumference 2πr and simplify fractions.
  • Identify the angle at a circle's centre.

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

Quick readiness check

How many radians make one full turn?

What you will learn

  • Explain the definition of one radian.
  • Convert degrees and radians, retaining exact multiples of π where possible.

A radian compares two lengths

One radian is the angle at the centre subtended by an arc whose length equals the radius. For an arc of length s on a circle of radius r > 0, its central angle is θ = s/r radians. The ratio is independent of circle size: scaling both lengths by the same factor preserves the angle. Radians do not mean a number of degrees with a different label.

A complete circumference has length 2πr, so one full turn is (2πr)/r = 2π radians = 360°. Half a turn is π radians = 180°. An arc of length 12 cm on a circle of radius 8 cm subtends 12/8 = 3/2 radians, about 85.9°. Use the radius, not the diameter, in this definition.

Convert the value before using it

Convert degrees to radians by multiplying by π/180. Convert radians to degrees by multiplying by 180/π. Thus 225° = 5π/4 radians and 7π/6 radians = 210°. Keep the exact multiple of π in an algebraic answer; for an angle without π, such as 1.2 radians, use a calculator for its degree approximation.

Check calculator mode before evaluating a trigonometric function. sin(π/6) in radian mode and sin(30°) in degree mode both equal 1/2. Typing π/6 in degree mode evaluates a different angle. Negative angles describe the opposite direction of rotation, and angles larger than a full turn are valid; the conversion factor does not change.

Pause and explain

Convert 150° to an exact radian value.

Put the idea to work

Worked example

Convert 225° to radians and 7π/6 radians to degrees.

Show the worked solution
  1. Use π radians = 180°, so 225° becomes 225π/180.
  2. Simplify the fraction to obtain 5π/4 radians.
  3. For the reverse conversion, (7π/6)(180/π) = 210°.

Answer 225° = 5π/4 radians; 7π/6 radians = 210°.

Try a topic test

Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.

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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

Convert 135° to radians and π/8 radians to degrees.

Give me a hint

Use the two reciprocal conversion factors and cancel π in the second calculation.

Compare my reasoning
  1. Write 135π/180 and simplify by dividing numerator and denominator by 45.
  2. This gives 3π/4 radians.
  3. For π/8 radians, (π/8)(180/π) = 22.5°.

135° = 3π/4 radians; π/8 radians = 22.5°.

Look for these in your work

  • Use the correct conversion direction.
  • Retain an exact radian answer and state the degree unit.
2 · Independent

Convert −240° to radians and 11π/6 radians to degrees.

Give me a hint

A signed angle uses the same conversion rule; keep its sign.

Compare my reasoning
  1. Multiply −240 by π/180 to obtain −4π/3.
  2. Multiply 11π/6 by 180/π to obtain 330°.
  3. The negative sign indicates clockwise rotation under the usual positive anticlockwise convention.

−240° = −4π/3 radians; 11π/6 radians = 330°.

Look for these in your work

  • Preserve the direction sign.
  • Simplify exact multiples of π correctly.
3 · Transfer

A circular wheel of radius 10 cm turns through an arc length 7 cm. Find its angle in radians, then in degrees to one decimal place.

Give me a hint

The angle is arc length divided by radius; only the second answer needs approximation.

Compare my reasoning
  1. Use θ = 7/10 = 0.7 radians.
  2. Convert to degrees: 0.7 × 180/π ≈ 40.1070°.
  3. Round the degree result at the end to 40.1°; the arc length is not a straight-line chord.

0.7 radians; 40.1°.

Look for these in your work

  • Use the defining ratio with the radius.
  • Round only after converting the angle.

Common mistakes

  • A radian is defined using an arc and the radius, not a chord or diameter.
  • Do not replace θ by a degree value in a radian formula.
  • Retain π until the final numerical approximation.
Recall without your notes

What can you explain now?

An arc is 12 cm long and its circle has radius 8 cm. Find the central angle in radians and degrees.

Compare with the explanation

3/2 radians, approximately 85.9°.

The defining ratio is 12/8. Multiply 3/2 by 180/π for the degree value.

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

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Make it stick

Explain why one full turn is 2π radians, then convert one positive and one negative angle without using a memorised decimal for π.

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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