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A Level · Cambridge (CIE) · 9709

Trigonometric equations

Find every solution in an interval by considering signs and symmetry.

Pure Mathematics 1 · Trigonometric equations revision · 1.5

Pure Mathematics 1 (Paper 1) · Mathematics 9709, 2026–2027. This revision lesson focuses on complete solution sets in specified intervals, not general forms of solution or the whole trigonometry syllabus. Written solutions use self-check criteria, not an official mark scheme. Specialist accuracy review has not been completed.

Before you begin

  • Use exact sine and cosine values for 30°, 60° and their radian equivalents.
  • Factor a product equal to zero without dividing by a possibly zero factor.

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

Quick readiness check

Solve sin θ = −√3/2 for 0° ≤ θ < 360°. Which set includes every solution?

What you will learn

  • Find a reference angle.
  • Select all quadrants consistent with the equation.
0°90°180°270°360°sin xcos x1−10
Sine and cosine over 0° to 360°. Sine starts at zero, reaches one at 90°, zero at 180°, minus one at 270° and zero at 360°. Cosine starts at one and has its minimum at 180°. Both have period 360°.

Isolate the trigonometric ratio

Rearrange the equation until a single sine, cosine or tangent is isolated. Find a reference angle using an exact value or an inverse function. An inverse calculator result gives one value, not automatically every solution in the requested interval.

Sine is positive in quadrants I and II, cosine in I and IV, and tangent in I and III. Use the sign together with the reference angle, then check the original equation.

Respect the interval and units

A solution outside the stated interval must be excluded. Endpoints need particular care because an inclusive interval may contain both 0° and 360°. Check whether the question uses degrees or radians before forming solutions.

If sin θ=sin α, solutions repeat with period 360° (or 2π radians).

Sources for this lesson

Pause and explain

The cosine graph has value 1 at 0° and 360°. For cos x = 1 on 0 ≤ x ≤ 2π radians, which solution set is complete?

Put the idea to work

Worked example

Solve 2cos θ=−1 for 0°≤θ≤360°.

Show the worked solution
  1. Rearrange to cos θ=−1/2, with reference angle 60°.
  2. Cosine is negative in quadrants II and III.
  3. The angles are 180°−60° and 180°+60°.

Answer 120° and 240°

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

Solve 2 sin θ + √3 = 0 for −π ≤ θ ≤ π. State exact radian answers and explain why a positive reference angle is not automatically a solution.

Give me a hint

Isolate sine. Locate the two negative-sine positions within the supplied interval; use a full-turn shift if starting with positive angles.

Compare my reasoning
  1. Rearrange to sin θ = −√3/2. Its reference angle is π/3, but sin(π/3) is positive and does not satisfy this equation.
  2. In one positive turn the corresponding angles are 4π/3 and 5π/3. Subtract 2π from each to place them in the requested interval: −2π/3 and −π/3.
  3. Both have sine −√3/2. At −π, 0 and π the sine is 0, so these points are not solutions. The negative half-turn contains exactly the two required crossings.

θ = −2π/3 or −π/3.

Look for these in your work

  • Isolate the ratio and keep its negative sign.
  • Convert all selected angles to the supplied interval and units.
  • Check both original substitution and completeness.
2 · Independent

Solve cos(2x) = −√3/2 for 0 ≤ x < 2π. Show the interval for u = 2x before finding x.

Give me a hint

The cosine input covers two full turns. Find every permitted u first, then divide all values by 2.

Compare my reasoning
  1. Let u = 2x. Then 0 ≤ u < 4π, so searching only 0 ≤ u < 2π would miss a full turn.
  2. In the first turn, negative cosine with reference angle π/6 gives u = 5π/6 and 7π/6. In the second turn add 2π to obtain 17π/6 and 19π/6.
  3. Divide each angle by 2: x = 5π/12, 7π/12, 17π/12, 19π/12. All lie in the original half-open interval and each doubled angle gives the required cosine.

x = 5π/12, 7π/12, 17π/12 or 19π/12.

Look for these in your work

  • Transform the interval with the argument.
  • Include both negative-cosine positions in both turns.
  • Divide every angle and filter in the original interval.
3 · Transfer

Solve sin x(2 cos x + 1) = 0 for −π ≤ x ≤ π. A learner divides by sin x before solving. Identify exactly which solutions this loses.

Give me a hint

Use the zero-product rule and solve both factors separately; retain included endpoints even when they share a unit-circle direction.

Compare my reasoning
  1. The product is zero when sin x = 0 or 2 cos x + 1 = 0. Dividing by sin x would exclude the entire first case.
  2. For sin x = 0, the allowed values are −π, 0 and π. For cos x = −1/2, the allowed values are −2π/3 and 2π/3.
  3. Combine and order the values: −π, −2π/3, 0, 2π/3, π. Each makes at least one original factor zero. Division by sin x loses −π, 0 and π.

x = −π, −2π/3, 0, 2π/3 or π. Division loses −π, 0 and π.

Look for these in your work

  • Preserve both branches of the zero-product rule.
  • Find all values in each branch, including endpoints.
  • Name the lost values and check them in the original product.

Common mistakes

  • One inverse-trig output is not a complete solution set.
  • Do not mix radian and degree values.
Recall without your notes

What can you explain now?

Solve sin θ=1/2 for 0≤θ≤2π.

Compare with the explanation

π/6 and 5π/6

The positive sine solutions lie in quadrants I and II.

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

Your review choice appears on Today. Sign in to sync it across devices.

Make it stick

Explain why an inverse-function output may be only one solution. Describe how to transform the interval when the input is 2x, and why dividing by sin x needs a separate zero-sine case. Use a sketch to distinguish the endpoints 0 and 2π from the half-turn 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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