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.
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.
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?
Worked example
Use four equal intervals to estimate ∫ from 0 to 2 of x² dx by the trapezium rule.
Show the worked solution
- Four intervals give width h = 1/2 and five inputs 0, 1/2, 1, 3/2, 2.
- Their ordinates are 0, 1/4, 1, 9/4 and 4; weight the endpoints once and interior values twice.
- 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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Test this topic →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.
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
- Take h = 1, endpoint ordinates 1 and 10, and interior ordinates 2 and 5.
- T = (1/2)[1 + 10 + 2(2 + 5)].
- 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.
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
- The ordinates are 1, e^(1/2) and e, with h = 1/2.
- T = (1/4)[1 + e + 2e^(1/2)].
- 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.
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
- The first strip estimates 1(2 + 4)/2 = 3 litres.
- The second estimates 2(4 + 6)/2 = 10 litres.
- 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.
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.