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IB Math AI SL · Topic 5

Calculus

Applied, not abstract: differentiation finds best answers to word problems, integration measures areas under them, and the trapezoidal rule estimates what cannot be integrated neatly. The mathematics is short; the justification around it is everything.

What the syllabus covers

Eight assessed sections, SL 5.1 to 5.8.

SL 5.1Limits and the derivativeThe limit concept introduced informally; the derivative interpreted as a gradient function and as a rate of change.
SL 5.2Increasing and decreasingReading f′(x) > 0, f′(x) = 0 and f′(x) < 0 off a graph, and what each means for the function itself.
SL 5.3Differentiating polynomialsFunctions of the form f(x) = axⁿ + bxⁿ⁻¹ + …, where all exponents are integers.
SL 5.4Tangents and normalsTheir equations at a given point — gradient from the derivative, line through the point with substitution shown.
SL 5.5IntegrationAnti-differentiation of axⁿ-type functions (n ≠ −1); boundary conditions to pin down the constant; definite integrals by technology; area under a curve above the x-axis.
SL 5.6Stationary pointsSolving f′(x) = 0; identifying local maxima and minima among the solutions.
SL 5.7Optimisation in contextWord problems that run model → differentiate → solve → interpret, with units and a conclusion in terms of the scenario.
SL 5.8Trapezoidal ruleApproximating areas under curves when integration is not available — table of values, interval width h, weighted sum.

How it is examined

Optimisation word problems close Paper 2 modelling chains — the calculus is three lines, the framing and justification carry the rest.

  • Optimisation questions arrive fully dressed in context — maximum profit, minimum fencing — requiring a variable defined, a function constructed, and a stationary point found and classified.

  • Finding f′(x) = 0 earns method marks; asserting 'this is the maximum' without checking nature costs the reasoning mark.

  • Tangents and normals are examined as equations through a given point, with substitution shown.

  • Trapezoidal-rule questions provide tables of values; the interval width h is where silent errors live.

  • The booklet gives exactly three things here: the power rule for differentiating xⁿ, the power rule for integrating it, and the trapezoidal rule itself. Nothing about optimisation appears anywhere in it — the setup is entirely yours to construct.

  • The second-derivative test is not part of the SL syllabus, so a stationary point is classified by a sign-change argument around it — values of f′ either side, or the graph on your GDC.

A marked question, mark by mark

An optimisation chain showing where the justification mark separates complete answers from near-complete ones.

Get the justification habit, on fresh exercises

Unlimited generated optimisation, tangents and area questions across SL 5.1–5.8 — each returned mark by mark, so the missing classification or unit becomes visible immediately, not on results day.

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New to the codes? M1, A1, R1 explained →

Optimisation · SL 5.6 / 5.7

Seed 6A52

A farmer builds a rectangular enclosure against an existing wall using 60 m of fencing for the other three sides.

  1. (a)Show that the area is A(x) = x(60 − 2x), where x is the depth.
  2. (b)Find the dimensions that maximise the area, justifying that it is a maximum.

Your work

A(x) = x(60 − 2x)

A′(x) = 60 − 4x = 0 → x = 15

dimensions 15 m × 30 m, A = 450 m²

Marks awarded

  • M1Mark awarded. Differentiated and solved A′(x) = 0 correctly.
  • A1Mark awarded. x = 15 m, maximum area 450 m².
  • R1Mark not awarded. Maximum asserted, never justified — no second-derivative check or sign-change argument shown.

Where marks are actually lost

Calculus answers end with sentences, not numbers.

Stopping at the solution of f′(x) = 0 without classifying the stationary point.

CostThe reasoning mark exists precisely to distinguish maxima from minima — unearned by assertion.

Final answers without units or a closing sentence in the problem's terms.

Cost'x = 15' is a coordinate; 'the enclosure should be 15 m deep' is the answer the question asked for.

Setting the wrong interval width h in trapezoidal-rule approximations.

CostEvery ordinate is then weighted incorrectly and the estimate collapses.

Sign slips differentiating negative-coefficient polynomials.

CostFollow-through protects later marks only when the written method stays consistent and readable.