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A Level · Cambridge (CIE) · 9709

Calculate geometric first-success probabilities

Use the single required sequence of failures followed by success, without a binomial order factor.

Statistics 1 · Discrete random variables · 5.4
Exam scope and assessment notes

Probability & Statistics 1 (Paper 5, AS & A Level) · Mathematics 9709, 2026–2027, section 5.4. A probability table needs non-negative probabilities summing to one. E(X) is a probability-weighted mean; Var(X) = E(X²) − [E(X)]², with squared units. A binomial model B(n,p) counts successes in a fixed non-negative integer number n of independent two-outcome trials with a constant success probability p. A geometric model Geo(p) counts the trial of the first success, starting at 1, with independent trials and a constant 0 < p ≤ 1. Formula proofs are not required. The expectation of the geometric distribution is required; its variance is not required by section 5.4. Normal approximations belong to section 5.5 and are not used here. Exact model probabilities do not establish real-world independence or constant rates without evidence. Written practice is self-checked; human subject-teacher review remains pending.

Before you begin

  • Define a geometric first-success variable.
  • Multiply probabilities along independent paths.

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

Quick readiness check

How many failures precede first success on trial five?

What you will learn

  • Calculate P(T = r) = (1 − p)ʳ⁻¹p.
  • Explain why the first-success event has exactly one success placement.

Failures must come before the success

For T ~ Geo(p), the event T = r requires r − 1 failures followed by one success. Thus P(T = r) = (1 − p)ʳ⁻¹p for integer r ≥ 1. The successful trial is fixed at the final position; there is no choice of success positions and no combination factor.

For p = 1/4 and r = 3, the only permitted string is failure, failure, success, with probability (3/4)²(1/4) = 9/64. A string containing a success on trial one or two would have stopped earlier. Exactly one success among three fixed trials is a different binomial event and would allow three positions.

Use the variable’s support

For p = 1/5, first success on trial five has four preceding failures, giving (4/5)⁴(1/5) = 256/3125 = 0.08192. Counting five failures would move the success to trial six. For p = 1/3, P(T = 1) = 1/3, while P(T = 4) = (2/3)³(1/3) = 8/81.

T cannot equal zero or a non-integer, so those events have probability zero. At p = 1 the first trial succeeds with certainty, giving probability one at T = 1 and zero for all later values. Address this endpoint directly if a calculator presents an ambiguous zero-power expression. Formula proofs are not required; the event definition provides the needed reasoning.

Pause and explain

For T ~ Geo(1/4), what is P(T = 3)?

Put the idea to work

Worked example

T ~ Geo(1/4). Find P(T = 3).

Show the worked solution
  1. First success on trial three requires two failures followed by success.
  2. Multiply their probabilities: (3/4)(3/4)(1/4).
  3. The result is 9/64, with no order factor.

Answer P(T = 3) = 9/64.

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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

T ~ Geo(1/4). Find P(T = 3).

Give me a hint

Write the unique two-failure, one-success path.

Compare my reasoning
  1. Trials one and two must fail, each with probability 3/4.
  2. Trial three must succeed with probability 1/4.
  3. Multiply (3/4)²(1/4) = 9/64.

9/64.

Look for these in your work

  • Use two preceding failures.
  • Avoid an extra success-position factor.
2 · Independent

T ~ Geo(1/5). Find the probability of first success on trial five.

Give me a hint

The first four trials fail and the fifth succeeds.

Compare my reasoning
  1. Use four failure factors, each 4/5.
  2. Multiply by the final success factor 1/5.
  3. (4/5)⁴(1/5) = 256/3125 = 0.08192.

256/3125 = 0.08192.

Look for these in your work

  • Match the exponent to the preceding failures.
  • State the exact first-success event.
3 · Transfer

T ~ Geo(1/3). Compare P(T = 1) and P(T = 4), explaining the exponents.

Give me a hint

The first case needs no preceding failure; the second needs three.

Compare my reasoning
  1. P(T = 1) = 1/3 because success occurs immediately.
  2. P(T = 4) = (2/3)³(1/3) = 8/81.
  3. The failure exponents are zero and three because the successful trial is included in T.

P(T = 1) = 1/3; P(T = 4) = 8/81.

Look for these in your work

  • Keep the first-trial convention consistent.
  • Explain the number of preceding failures.

Common mistakes

  • Use r − 1 failures, not r.
  • Do not add a combination factor to the unique first-success path.
  • First success on trial r differs from one success among r fixed trials.
Recall without your notes

What can you explain now?

Why is there no 3C1 factor for first success on trial three?

Compare with the explanation

The only allowed success position is trial three.

Any earlier success changes the first-success trial and leaves the event.

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Make it stick

First success on trial r means exactly r − 1 initial failures and a final success in a single permitted order.

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.

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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.

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