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.
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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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.
- (a)Show that the area is A(x) = x(60 − 2x), where x is the depth.
- (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.
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