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

Expand rational powers of one plus x with correct coefficients

Extend the binomial expansion to rational exponents, keeping signs, factorial divisors and its strict validity condition.

Pure Mathematics 3 · Algebra · 3.1
Exam scope and assessment notes

Pure Mathematics 3 (Paper 3, A Level) · Mathematics 9709, 2026–2027, section 3.1. Paper 1 knowledge is assumed. Shared modulus, polynomial division, factor and remainder lessons are linked from both sections 2.1 and 3.1. Paper 3 adds partial fractions and rational-power binomial expansions. Partial-fraction numerators may have the same degree as the denominator, but higher-degree numerators are excluded. General-term questions for the rational binomial expansion are not included. Use the stated strict validity condition. Written practice is self-checked; human subject-teacher review remains pending.

Before you begin

  • Expand a positive-integer binomial and calculate factorials.
  • Work with signed fractions, index notation and powers of a small input.

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

Quick readiness check

What is 3!?

What you will learn

  • Use the first terms of (1 + x)ⁿ for rational n, including negative integers and fractional powers.
  • Distinguish a truncated approximation from an exact polynomial and state |x| < 1.
A cubic approximation is not the exact root-0.8-0.400.40.80.511.5xyBlue: exact √(1 + x)Red dashed: cubic truncation
The solid blue curve is the exact function √(1 + x); the dashed red curve is its cubic truncation 1 + x/2 − x²/8 + x³/16. Both are drawn only for −0.8 ≤ x ≤ 0.8, inside the strict expansion interval |x| < 1. At x = −0.75, the exact value is 0.5 and the cubic value is 541/1024, approximately 0.52832. Their visible separation shows that four terms are an approximation, not the exact function. This graph gives no guaranteed bound on the omitted terms. Horizontal and vertical scales differ, and both curves continue beyond the displayed window.

Use descending exponent factors and factorial divisors

For rational n and |x| < 1, the expansion begins (1 + x)ⁿ = 1 + nx + [n(n − 1)/2!]x² + [n(n − 1)(n − 2)/3!]x³ + … . The successive exponent factors continue to decrease by 1; they are not all equal to n. The factorial divisors are 2, 6, 24 and so on. For a non-negative integer n the expansion eventually terminates, but a negative integer or non-integer rational exponent generally gives an infinite series.

For n = −2, the coefficients through x³ are 1, −2, 3 and −4: (1 + x)⁻² = 1 − 2x + 3x² − 4x³ + … , valid for |x| < 1. The x² coefficient comes from (−2)(−3)/2 = 3; the x³ coefficient comes from (−2)(−3)(−4)/6 = −4. Retain the alternating signs rather than copying a positive-integer Pascal row.

Keep the series condition and approximation separate

For n = 1/2, the first four terms are 1 + x/2 − x²/8 + x³/16. Thus √(1 + x) is approximately this cubic for small x within |x| < 1, but the cubic is not the exact square-root function. At x = −3/4, the exact square root is 1/2 and the cubic is 541/1024, approximately 0.52832. The visible difference is a reminder that being inside the validity interval alone does not guarantee a requested numerical accuracy after only four terms.

Write an ellipsis or an explicit truncation when reporting an infinite expansion. If asked for terms up to x³, include every term through that power and state |x| < 1. The strict condition describes the syllabus's guaranteed expansion interval; it does not say that the original function is undefined at every endpoint or beyond the interval. Finding a general term for these rational expansions is outside the stated requirements.

Pause and explain

Which coefficient multiplies x² in (1 + x)¹ᐟ²?

Put the idea to work

Worked example

Expand (1 + x)⁻² through the x³ term and state the condition.

Show the worked solution
  1. Substitute n = −2 into the descending-factor formula.
  2. Compute coefficients −2, (−2)(−3)/2 = 3, and (−2)(−3)(−4)/6 = −4.
  3. Retain the ellipsis because this negative-integer expansion is infinite, with |x| < 1.

Answer 1 − 2x + 3x² − 4x³ + … , valid for |x| < 1.

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

Expand (1 + x)⁻¹ through x³ and state the validity interval.

Give me a hint

Use exponent −1 with factors −1, −2 and −3, keeping the factorial denominators.

Compare my reasoning
  1. The linear coefficient is −1 and the quadratic coefficient is (−1)(−2)/2 = 1.
  2. The cubic coefficient is (−1)(−2)(−3)/6 = −1.
  3. The expansion is 1 − x + x² − x³ + … , for −1 < x < 1; the negative exponent does not make the series terminate.

1 − x + x² − x³ + … , |x| < 1.

Look for these in your work

  • Compute signs and factorial divisors correctly.
  • State the strict interval and indicate omitted higher powers.
2 · Independent

Expand (1 + x)⁻¹ᐟ² through x³.

Give me a hint

Use n = −1/2 and subtract 1 from each successive exponent factor.

Compare my reasoning
  1. The linear coefficient is −1/2; the quadratic coefficient is (−1/2)(−3/2)/2 = 3/8.
  2. The cubic coefficient is (−1/2)(−3/2)(−5/2)/6 = −5/16.
  3. Write 1 − x/2 + 3x²/8 − 5x³/16 + … , keeping the condition |x| < 1.

1 − x/2 + 3x²/8 − 5x³/16 + … , |x| < 1.

Look for these in your work

  • Use the descending fractional factors.
  • Keep exact coefficients and the validity condition.
3 · Transfer

Expand (1 + x)²ᐟ³ through x³, explaining why the quadratic and cubic coefficients have different signs.

Give me a hint

The descending factors after 2/3 are −1/3 and −4/3.

Compare my reasoning
  1. The linear coefficient is 2/3, and (2/3)(−1/3)/2 gives −1/9 for x².
  2. The two negative factors make (2/3)(−1/3)(−4/3)/6 = 4/81 positive for x³.
  3. Thus the expansion is 1 + 2x/3 − x²/9 + 4x³/81 + … , valid for |x| < 1.

1 + 2x/3 − x²/9 + 4x³/81 + … , |x| < 1.

Look for these in your work

  • Derive each sign from the actual exponent factors.
  • Report all four terms as an infinite-series truncation.

Common mistakes

  • Decrease the exponent factor at each stage; n³ is not the cubic numerator.
  • Include the factorial divisor and all signs.
  • A finite truncation is not exactly equal to the full function for arbitrary non-zero x.
Recall without your notes

What can you explain now?

Find the coefficient of x² in √(1 + x).

Compare with the explanation

−1/8.

With n = 1/2, n(n − 1)/2 = (1/2)(−1/2)/2 = −1/8.

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

Derive the first four coefficients for a rational exponent, explain which integer powers terminate, and separate validity from the accuracy of a short truncation.

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.