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A Level · Cambridge (CIE) · 9709

Apply the trapezium rule with correct spacing and endpoint weights

Build straight-sided strips from the ordinates, distinguish intervals from points and report numerical integration as an estimate.

Pure Mathematics 2 · Integration · 2.5
Exam scope and assessment notes

Pure Mathematics 2 (Paper 2, AS only) · Mathematics 9709, 2026–2027. Paper 1 integration and Paper 2 derivatives and identities are assumed. The AS route using Papers 1 + 2 cannot be carried forward to A Level. Section 2.5 covers reverse differentiation of linear-composite exponentials, reciprocals and trigonometric functions, integration using trigonometric relationships, and trapezium estimates with sketch-based over/under-estimation. The general method of integration by substitution is not required here. Use radians for trigonometric calculus. Written practice is self-checked.

Before you begin

  • Find the area of a trapezium and evaluate a function at supplied inputs.
  • Distinguish signed definite integrals from non-negative geometric area.

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

Quick readiness check

What is the area of a trapezium with parallel lengths p and q separated by width h?

What you will learn

  • Use n equal intervals and n + 1 ordinates to calculate a trapezium estimate.
  • Apply correct endpoint weights, interval widths and units to functions or supplied measurements.
Four strips share five ordinates0123400.511.52h = 1/2xyEstimate: 11/4 · Exact integral: 8/3
Four equal intervals of width 1/2 on [0, 2] require five ordinates of y = x²: 0, 1/4, 1, 9/4 and 4. The straight dashed red chords bound the pale shaded trapezia; the solid blue curve is the original function. Interior ordinates are shared by two strips, whereas the two outer endpoints each occur once. The trapezium estimate is 11/4 = 2.75, above exact integral 8/3 because the upward-bending curve lies below its chords. The shading depicts the approximate signed area in this non-negative example.

Join neighbouring ordinates with straight segments

To estimate ∫ from a to b of f(x) dx, split the interval into n equal widths h = (b − a)/n. The n + 1 ordinates are y₀ = f(a), y₁ = f(a + h), …, yₙ = f(b). A strip between two neighbours contributes h(yᵢ + yᵢ₊₁)/2. Adding all strips gives T = (h/2)[y₀ + yₙ + 2(y₁ + … + yₙ₋₁)]. Each interior ordinate belongs to two strips, while each outer endpoint belongs to one.

For f(x) = x² on [0, 2] with four intervals, h = 1/2 and the five ordinates are 0, 1/4, 1, 9/4 and 4. The estimate is (1/4)[0 + 4 + 2(1/4 + 1 + 9/4)] = 11/4 = 2.75. Four intervals require five values, not four. With two intervals the estimate is 3; a changed partition gives a changed estimate, not a changed exact integral.

Use the actual measurement gaps and the stated units

The equally spaced formula needs one common width. If measurement times are 0, 1 and 3 minutes with rates 2, 4 and 6 litres per minute, use two separate trapezia: 1(2 + 4)/2 + 2(4 + 6)/2 = 13 litres. The middle rate is shared, but the unequal widths mean the common-h formula cannot be applied unchanged. The estimate assumes straight interpolation between measurements; the data alone do not prove the actual rate followed those lines.

Negative ordinates contribute negative signed strips. This numerical rule estimates a signed integral, so taking absolute values of all ordinates changes the question. For geometric area or distance, identify sign changes and split the relevant regions before estimating their magnitudes. Multiplying a rate ordinate by its time width produces an accumulated quantity. If power is in kW, the time width must be in hours to report energy in kWh. Keep unrounded function values through the calculation and round the final estimate as requested.

Pause and explain

For four intervals on [0, 2], which inputs are needed?

Put the idea to work

Worked example

Use four equal intervals to estimate ∫ from 0 to 2 of x² dx by the trapezium rule.

Show the worked solution
  1. Four intervals give width h = 1/2 and five inputs 0, 1/2, 1, 3/2, 2.
  2. Their ordinates are 0, 1/4, 1, 9/4 and 4; weight the endpoints once and interior values twice.
  3. T = (1/4)[0 + 4 + 2(1/4 + 1 + 9/4)] = 11/4 = 2.75, an estimate of the integral.

Answer Trapezium estimate 11/4 = 2.75 using four intervals and five ordinates.

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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

At x = 0, 1, 2, 3, the ordinates are 1, 2, 5, 10. Use the trapezium rule to estimate the integral over [0, 3].

Give me a hint

There are three equal intervals of width 1 and two interior ordinates.

Compare my reasoning
  1. Take h = 1, endpoint ordinates 1 and 10, and interior ordinates 2 and 5.
  2. T = (1/2)[1 + 10 + 2(2 + 5)].
  3. The estimate is 25/2 = 12.5; report it as a trapezium estimate unless straight interpolation is explicitly the exact model.

Trapezium estimate 25/2 = 12.5.

Look for these in your work

  • Infer the interval count and width from the supplied inputs.
  • Weight endpoints and interior values correctly and label the result as an estimate.
2 · Independent

Use two equal intervals to estimate ∫ from 0 to 1 of eˣ dx. Keep exact ordinate values until the end and give the result to three significant figures.

Give me a hint

Evaluate eˣ at 0, 1/2 and 1; then use width 1/2.

Compare my reasoning
  1. The ordinates are 1, e^(1/2) and e, with h = 1/2.
  2. T = (1/4)[1 + e + 2e^(1/2)].
  3. The final estimate is approximately 1.75 to three significant figures; this does not equal the exact integral e − 1.

T = (1 + e + 2e^(1/2))/4 ≈ 1.75, a trapezium estimate.

Look for these in your work

  • Use three ordinates for the two requested intervals.
  • Retain exact values before rounding and distinguish estimate from exact integral.
3 · Transfer

Measured inflow rates at 0, 1 and 3 minutes are 2, 4 and 6 litres per minute. Estimate the added volume using separate trapezia and explain why a single common-width formula is unsuitable.

Give me a hint

Use the two actual time gaps 1 minute and 2 minutes.

Compare my reasoning
  1. The first strip estimates 1(2 + 4)/2 = 3 litres.
  2. The second estimates 2(4 + 6)/2 = 10 litres.
  3. The total estimate is 13 litres; unequal gaps require separate widths, and straight interpolation is an approximation to an unknown actual rate.

Estimated increase 13 litres; widths 1 and 2 minutes must be used separately.

Look for these in your work

  • Use each actual measurement gap and rate-times-time units.
  • State the straight-interpolation assumption without claiming an exact observed total.

Common mistakes

  • n intervals require n + 1 function values.
  • Multiply by h/2 and double only the interior ordinates.
  • Check spacing, sign, units and rounding before interpreting an estimate.
Recall without your notes

What can you explain now?

How many ordinates are needed for six equal trapezium intervals?

Compare with the explanation

Seven ordinates.

Every interval needs two neighbouring endpoints, with adjacent strips sharing their interior endpoints.

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Make it stick

Derive the endpoint and interior weights by adding neighbouring strips. Explain why intervals and ordinates have different counts, and why unequal gaps cannot share an unchanged common width.

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.

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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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