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A Level · Cambridge (CIE) · 9709

Solve quadratic trigonometric equations and reject invalid ratios

Use an identity to form a quadratic in one ratio, check that its roots are possible and recover every permitted angle.

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

  • Solve quadratic equations and use sin² x + cos² x = 1.
  • Find all simple trigonometric solutions in a supplied interval.

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

Quick readiness check

The substitution u = cos x gives u = 3/2 or u = −1/2. Which roots can give real angles?

What you will learn

  • Turn an equation in trigonometric ratios into a quadratic in one variable.
  • Reject impossible ratio values and find all remaining angles in the stated interval.

A quadratic root is a ratio before it is an angle

For 2 sin² x − 3 sin x + 1 = 0, let u = sin x and solve 2u² − 3u + 1 = (2u − 1)(u − 1) = 0. The ratio values are u = 1/2 or 1, both allowed because sine lies in [−1, 1]. On 0° ≤ x ≤ 360°, they give x = 30°, 150° and 90° respectively. List distinct angles once and check them in the original equation.

Dividing by sin x or cos x can remove legitimate zero-ratio solutions. For example sin x(2 sin x − 1) = 0 requires both sin x = 0 and sin x = 1/2. On 0° ≤ x ≤ 360°, its roots are 0°, 30°, 150°, 180° and 360°. Factor first and examine each factor, rather than dividing by a quantity that might be zero.

Use an identity when two ratios occur

For 3 sin² x + 2 cos x − 1 = 0, replace sin² x by 1 − cos² x and let u = cos x. The equation becomes 3u² − 2u − 2 = 0, giving u = (1 ± √7)/3. The plus root is greater than 1 and is impossible for a real cosine. The minus root is about −0.548584 and produces two angles in a full turn. Preserve its exact value until the final inverse calculation.

For tan² x − 3 tan x + 2 = 0, factor to obtain tangent values 1 and 2. Tangent is not bounded by [−1, 1], so 2 is valid; instead check its excluded inputs and the requested interval. On 0° ≤ x < 180°, the answers are 45° and approximately 63.4°. A range restriction belongs to a particular ratio, rather than every trigonometric function.

Pause and explain

Why is dividing sin x(2 sin x − 1) = 0 by sin x unsafe?

Put the idea to work

Worked example

Solve 2 sin² x − 3 sin x + 1 = 0 for 0° ≤ x ≤ 360°.

Show the worked solution
  1. Let u = sin x, then factor (2u − 1)(u − 1) = 0.
  2. Both u = 1/2 and u = 1 lie in the sine range; the first gives 30° and 150°, and the second gives 90°.
  3. Check all three in the original equation and list them in increasing order without duplicates.

Answer x = 30°, 90° or 150°.

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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

Solve 2 sin² x − 3 sin x + 1 = 0 on 0° ≤ x ≤ 360°.

Give me a hint

Solve a quadratic in u = sin x, then recover all angles for each allowed ratio.

Compare my reasoning
  1. Factor (2u − 1)(u − 1) = 0, giving u = 1/2 or 1.
  2. Sine 1/2 gives 30° and 150°; sine 1 gives 90° within this interval.
  3. Substitute all three and list x = 30°, 90°, 150°; neither endpoint satisfies the equation.

x = 30°, 90° or 150°.

Look for these in your work

  • Find both allowed quadratic ratio roots.
  • Recover and check all angles for each root.
2 · Independent

Solve 2 cos² x − 5 cos x + 2 = 0 for 0 ≤ x < 2π.

Give me a hint

Let u = cos x, factor the quadratic and apply the cosine range before taking inverses.

Compare my reasoning
  1. Factor (2u − 1)(u − 2) = 0, giving u = 1/2 or 2.
  2. Reject u = 2 because a real cosine cannot exceed 1.
  3. The remaining equation cos x = 1/2 gives x = π/3 and 5π/3 in the specified interval.

x = π/3 or 5π/3; the cosine root 2 is rejected.

Look for these in your work

  • Separate algebraic roots from permitted cosine values.
  • Find both full-turn angles for the valid root.
3 · Transfer

Solve sin² x + cos x = 1 for 0° ≤ x ≤ 360°. Explain why dividing by cos x would lose answers.

Give me a hint

Replace sin² x with 1 − cos² x and factor without division.

Compare my reasoning
  1. Substitute the identity to get 1 − cos² x + cos x = 1, hence cos x(1 − cos x) = 0.
  2. For cos x = 0 the angles are 90° and 270°; for cos x = 1 they are the two included endpoints 0° and 360°.
  3. All four satisfy the original equation. Dividing by cos x would discard 90° and 270° because their cosine is zero.

x = 0°, 90°, 270° or 360°; division by cosine loses the two zero-cosine answers.

Look for these in your work

  • Use the squared identity and preserve both factors.
  • Include valid endpoints and explain the lost-root danger.

Common mistakes

  • Quadratic roots are ratio values; convert them into all permitted angles.
  • Do not divide by a ratio that could be zero and lose its solutions.
  • Reject sine or cosine values outside [−1, 1], but do not apply that restriction to tangent.
Recall without your notes

What can you explain now?

Solve sin x(2 sin x − 1) = 0 for 0° ≤ x ≤ 360°.

Compare with the explanation

x = 0°, 30°, 150°, 180° or 360°.

The zero factor supplies the three zeros, while the second factor supplies both angles with sine 1/2.

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

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Make it stick

Give a complete routine for a quadratic trigonometric equation: substitute, solve, range-check, recover angles, check domains and endpoints, then verify in the original equation.

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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