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A Level · Cambridge (CIE) · 9709

Use exact trigonometric values and related angles

Derive special-angle values from triangles and use a unit-circle position to determine signs and reference angles.

Section 1.5 · Trigonometry

Pure Mathematics 1 · Trigonometry · 1.5

Pure Mathematics 1 (Paper 1) · AS & A Level Mathematics 9709, 2026–2027. State angle units and find every solution in the supplied interval; general forms of solution are not required in section 1.5. Written practice is self-checked.

Before you begin

  • Use right-triangle trigonometric ratios and Pythagoras.
  • Convert special angles between degrees and radians.

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

What is sin 30° exactly?

What you will learn

  • Use exact sine, cosine and tangent values for 30°, 45° and 60°.
  • Determine exact related-angle values in any quadrant, including negative angles and extra turns.

Derive the table instead of rounding it

A right isosceles triangle with shorter sides 1 and 1 has hypotenuse √2, so sin 45° = cos 45° = √2/2 and tan 45° = 1. Halving an equilateral triangle of side 2 gives a 30°–60°–90° triangle with sides 1, √3 and 2. Thus sin 30° = 1/2, cos 30° = √3/2, tan 30° = √3/3; sin 60° = √3/2, cos 60° = 1/2, tan 60° = √3.

The equivalent radian angles are π/6, π/4 and π/3. An exact answer retains fractions and surds: 0.866 is an approximation to √3/2. Rationalising 1/√3 gives √3/3; the two forms are equal. Use tan θ = sin θ/cos θ only where cosine is non-zero.

A unit circle supplies the sign

The unit-circle point at angle θ has coordinates (cos θ, sin θ). In quadrant I both are positive; in II cosine is negative and sine positive; in III both are negative; in IV cosine is positive and sine negative. Tangent takes the sign of their quotient. A reference angle is the acute angle to the x-axis; it provides the magnitude, while the quadrant determines the sign.

At 150° the reference angle is 30° and the point is in quadrant II, so cos 150° = −√3/2 and sin 150° = 1/2. At 225°, both coordinates are −√2/2 and tangent is 1. At 330°, tan is −√3/3. Reduce an angle by whole turns before locating its quadrant: sin 390° = sin 30° = 1/2. Negative angles rotate clockwise, so sin(−45°) = −√2/2.

What is sin(5π/4) exactly?

Put the idea to work

Worked example

Find sin(2π/3), cos(2π/3) and tan(2π/3) exactly.

Show the worked solution
  1. 2π/3 = 120°, in quadrant II with reference angle 60°.
  2. Use the special-angle magnitudes and quadrant signs: sin = √3/2 and cos = −1/2.
  3. Divide sine by cosine to obtain tan = −√3.

Answer sin(2π/3) = √3/2; cos(2π/3) = −1/2; tan(2π/3) = −√3.

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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

Find sin 120°, cos 120° and tan 120° exactly.

Give me a hint

Identify the reference angle and quadrant before applying the special-angle values.

Compare my reasoning
  1. 120° is in quadrant II and its reference angle is 180° − 120° = 60°.
  2. Sine is positive √3/2 and cosine is negative −1/2.
  3. Their quotient gives tangent −√3.

sin 120° = √3/2; cos 120° = −1/2; tan 120° = −√3.

Look for these in your work

  • Select the correct reference angle.
  • Apply separate sine and cosine signs before dividing.
2 · Independent

Find sin(−π/4) and cos(7π/6) exactly.

Give me a hint

Locate a clockwise quarter-turn reference angle and a quadrant-III π/6 reference angle.

Compare my reasoning
  1. −π/4 is −45°, so its sine is negative with magnitude √2/2.
  2. 7π/6 = 210°, with reference angle π/6 in quadrant III.
  3. Cosine is negative there, giving −√3/2.

sin(−π/4) = −√2/2; cos(7π/6) = −√3/2.

Look for these in your work

  • Handle negative rotation without losing its sign.
  • Use the reference-angle cosine magnitude in the correct quadrant.
3 · Transfer

A point is 8 units from the origin at anticlockwise angle 5π/6 from the positive x-axis. Find its exact coordinates.

Give me a hint

Use (r cos θ, r sin θ) and check the quadrant and distance.

Compare my reasoning
  1. 5π/6 is 150°, in quadrant II, with cos = −√3/2 and sin = 1/2.
  2. Multiply both unit-circle coordinates by 8 to get (−4√3, 4).
  3. The squared distance is 48 + 16 = 64, confirming radius 8 and the expected negative x, positive y signs.

(−4√3, 4).

Look for these in your work

  • Scale both unit-circle coordinates by the radius.
  • Check the exact distance and quadrant signs.

Common mistakes

  • A reference angle gives magnitude; it does not decide the sign.
  • A rounded decimal is not an exact surd value.
  • Tangent is undefined when cosine is zero.
Recall without your notes

What can you explain now?

Find cos 150° and tan 330° exactly.

Compare with the explanation

cos 150° = −√3/2; tan 330° = −√3/3.

Both have reference angle 30°, but quadrant II makes cosine negative and quadrant IV makes tangent negative.

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

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Make it stick

Rebuild the 30°, 45° and 60° values from two triangles, then explain how to get a negative-angle or quadrant-III value without memorising another table.

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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