Learn in your language
Skip to content
A Level · Cambridge (CIE) · 9709

Use the chain rule for composite power functions

Identify the outer power and inner rule, multiply their derivatives and preserve the original real domain.

Section 1.7 · Differentiation

Pure Mathematics 1 · Differentiation · 1.7

Pure Mathematics 1 (Paper 1) · AS & A Level Mathematics 9709, 2026–2027. Use an informal understanding of limits; formal general differentiation from first principles and points of inflexion are not required in section 1.7. Written practice is self-checked.

Before you begin

  • Differentiate rational powers and elementary polynomials.
  • Understand function composition and real-domain restrictions.

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

Quick readiness check

In y = (3x + 2)⁴, what is the inner derivative?

What you will learn

  • Differentiate composite functions whose outer function is a rational power.
  • Use an inner derivative for a gradient or rate rather than treating a composite as a simple x power.

Differentiate the outside and multiply by the inside rate

If y = uⁿ and u = g(x), then dy/dx = n[g(x)]^(n − 1)g′(x), where the functions are differentiable. The inner derivative measures how fast its input changes with x. For y = (3x + 2)⁴, the outer derivative is 4u³ and the inner derivative is 3, so y′ = 12(3x + 2)³. Omitting 3 gives an incorrect gradient.

The inner rule need not be linear. For y = (x² + 1)³, the outer derivative is 3u² and g′(x) = 2x, giving y′ = 6x(x² + 1)². You can verify this by expanding the original polynomial and differentiating term by term. Do not differentiate each factor of the result as if a product were a sum; this lesson's method is the chain rule for the original composite.

A root or reciprocal adds a domain check

For y = √(x² + 4), rewrite the outer rule as u^(1/2). Since x² + 4 is always positive, y′ = (1/2)(x² + 4)^(−1/2)(2x) = x/√(x² + 4) for every real x. For y = √(5x + 1), the function is real for x ≥ −1/5 but its finite derivative 5/[2√(5x + 1)] requires x > −1/5.

For y = (4x + 3)^(−2), the function excludes x = −3/4. The derivative is −2(4x + 3)^(−3) × 4 = −8/(4x + 3)³ on that same domain. If the independent variable is time, the identical chain reasoning gives an instantaneous rate with the output units divided by seconds. State the variable explicitly instead of changing symbols halfway through a calculation.

Pause and explain

What is the derivative of (3x + 2)⁴?

Put the idea to work

Worked example

Differentiate y = (x² + 1)³ and check by expanding the original polynomial.

Show the worked solution
  1. Set u = x² + 1; the outer derivative is 3u² and the inner derivative is 2x.
  2. Multiply them to get y′ = 6x(x² + 1)².
  3. Expanding y gives x⁶ + 3x⁴ + 3x² + 1, whose derivative 6x⁵ + 12x³ + 6x equals the chain-rule result.

Answer y′ = 6x(x² + 1)² = 6x⁵ + 12x³ + 6x.

Try a topic test

Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.

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.

1 · Guided

Differentiate y = (2x − 1)⁵.

Give me a hint

Keep the inner bracket intact while reducing the outer power.

Compare my reasoning
  1. Set u = 2x − 1, so dy/du = 5u⁴.
  2. The inner derivative is du/dx = 2.
  3. Multiply to obtain dy/dx = 10(2x − 1)⁴, valid for every real x.

y′ = 10(2x − 1)⁴.

Look for these in your work

  • Identify the outer power and inner derivative separately.
  • Multiply their rates without losing the inner bracket.
2 · Independent

Differentiate y = 1/(4x + 3)² and state its excluded input.

Give me a hint

Rewrite the reciprocal as a negative power before using the chain rule.

Compare my reasoning
  1. Write y = (4x + 3)^(−2), with x ≠ −3/4.
  2. The outer derivative is −2(4x + 3)^(−3), and the inner derivative is 4.
  3. Thus y′ = −8/(4x + 3)³; the original excluded input remains excluded.

y′ = −8/(4x + 3)³, with x ≠ −3/4.

Look for these in your work

  • Apply a negative outer exponent correctly.
  • Retain the domain exclusion from the original reciprocal.
3 · Transfer

A supplied square-side model is L(t) = 2t² + 1 metres for t ≥ 0 seconds. Find the area rate at t = 2 seconds.

Give me a hint

The area is a composite A(t) = [L(t)]²; differentiate the side as well as the outer square.

Compare my reasoning
  1. A(t) = (2t² + 1)² square metres.
  2. The chain rule gives dA/dt = 2(2t² + 1)(4t) = 8t(2t² + 1).
  3. At t = 2, this is 8(2)(9) = 144 m²/s, not the side's rate 8 m/s.

dA/dt = 8t(2t² + 1); at t = 2, the area increases at 144 m²/s.

Look for these in your work

  • Model area as the square of the supplied side function.
  • Use both chain factors and distinguish area-rate units from side-rate units.

Common mistakes

  • The inner derivative is a multiplier, not an optional extra.
  • A root's real domain and finite-derivative domain may have different endpoints.
  • Preserve the independent variable and rate units in a time model.
Recall without your notes

What can you explain now?

Differentiate y = √(5x + 1) and state where the finite derivative is valid.

Compare with the explanation

y′ = 5/[2√(5x + 1)], for x > −1/5.

The factor 5 is the inner derivative; at the root boundary the derivative denominator is zero.

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

Your review choice appears on Today. Sign in to sync it across devices.

Make it stick

Explain the chain rule as an outside rate multiplied by an inside rate, then apply it to a root, a reciprocal and a non-linear inner function.

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.

StudyForA · Learn with understanding

Original teaching content and links to official sources. Check your exam year and qualification before using a resource.